(feat) add Library sidebar bookmarks

[ronso0]

Just recently I needed this again. I was doing a b2b session with a guest who had his music in various directories on a thumb drive. Switching between my and his tracks views was annoying.
Now it's just Alt+Up/Down 😁

The bookmark indicator alone is helpful if you want to only visually mark sidebar items, eg. some favorite History playlist or some directory in a huge list, without the next/prev functions.

Related feature request is #9912
Note: for easy testing I have included all fixes from #14556.

How to use

• press Alt+B to bookmark the selected sidebar item (or remove bookmark)
• press Alt+Up/Down to go to and highlight the previous/next bookmark.
  bookmarks are sorted by position in the tree, so Down really jumps to the next bookmark below.
  (note: missing bookmark items are skipped, but not removed from the list)
  -> this does not activate (load) the bookmark view
  -> useful for scrolling to a bookmark in order to drag tracks from the current view / from decks onto it
  -> press Enter to activate it

Quick demo, with and without activating the bookmark
(edit: auto-activation has now been disabled, see above)

Implementation

New struct `SidebarBookmark` holds all info to identify item: * data/label to find it in the tree * feature row, child level and parent row to sort bookmarks by position in the tree * feature row is also used to check only indices of the changed feature when the sidebar is updated

All bookmarked indices are stored QModelIndexList. This functions as quick lookup list for SidebarModel to paint the bookmarks.

Bookmarks and this index list are updated when the sidebar changed, eg. when playlists were added, removed etc. (when child tree models are rebuilt). By boomark data, we try to find the item in the updated tree, then store the new index and update the index list.
-- btw that was the real challenge here. If the sidebar is unchanged (ie. only labels (track count) are changed) it works by simply maintaining a QModelIndexList, but with potentially changing indices we need to store items' metadata to identify them later on and get their new indices.

New SidebarItemDelegate does the painting, and also detects left-click on Refresh icons.

Some new TreeItem members to help with BrowseFeature updates etc.

New extra feature / fix / slightly disruptive UI change

Recalling bookmarks in currently collapsed Computer trees causes a crash because the directory trees are rebuilt on expandation ¹.
Now, the child trees are only build on initial expandation. On next re-expandation we check if child directories changed.
If yes, a refresh icon is added to indicate changes.
Trees can be rebuild on demand, either by clicking the Refresh icon or with the existing "Refresh directory tree" context menu action.
Note that this is noticeable only if the if child directories changed, so it's actually not that disruptive IMO

TODO

• test with external features (iTunes, Banshee, Traktor ..)
• polish
• remove unneeded debug logs, qWarning -> qDebug

Known Issues

• (with (feat) Computer feature: use library data model if selected path is watched by library #14687 only) if there's a Quick Link bookmarked and another tree item with the same (absolute) path exist, and one is bookmarked, both are highlighted as Bookmarks

Nice to have

• make bookmarks persist. cfg? db?
  I'd go with the config. I'm lazy and a [Bookmarks] group would do, keys are the [feature] names and values are comma-separated ids/paths 🤷
• better styling than just a colored bar
• COs for bookmark, next, prev
• GUI controls to set and recall bookmarks?
  like hotcue buttons, click to set/recall, right-click to clear
  requires a Bookmark row or something..
• menu actions to bookmark items?

Footnotes

1. When switching to a bookmark in a collapsed tree, WLibrarySidebar calls QTreeView::scrollTo(index). This expands all the index' parents from current to top level. While doing this, QTreeView's expanded signal triggers BrowseFeature::onLazyChildExpandation which concurrently rebuilds the child tree, which emits signals to update the tree view and its model.
   Apparently we run into an QModelIndex or its TreeItem pointer in SidebarModel::parent that becomes invalid while we process it.
   Here's the backtrace ↩

[ronso0]

I discovered by chance QAbstractItemModel::match() after I wrote my own recursive match() function that checked label and/or data 😬 🤦‍♂️ 😆

[ronso0]

I think it's now safe to say this is ready for real-life testing.
I'll still occasionally check the commits and polish it and let you know when I consider it ready for review.

[ronso0]

Yeah, ready for review!
I've used this a lot and didn't notice issues so far.

[ronso0]

Bookmarks are now persistent (I really wanted to avoid the redundant steps when starting a session).
I opted fo the config way, simply because I was too lazy to dive into the database stuff, do schema upgrade etc.
Admittedly it's not the most beautiful solution but so far it seesm to work.
Note: only bookmarks are restored where the referenced item is available directly after start. Meaning bookmarks for lazily created tree items in Computer can currently not be restored.

[daschuer]

I have not tested yet, I just stumbled about the refresh icon in two ways. Maybe because I have not fully understand the workflow.

1. Why need the user be bothered with it at all?
2. Do you detect changes but not display them due to lazy loading? The established icon would be a dot overlay.
3. Or is it reload icon? Than a icon like in Firefox will also work.

[ronso0]

It's been a while since I did this, so I'm not entirely sure.

I tried to explain the main issue with recalling bookmarks in Computer here
TL;DR lazy tree rebuilding causes issues when programmtically going down / to a specific index in a collapsed tree

2. Do you detect changes but not display them due to lazy loading?

I compare the child count only (actual on disk vs. currently in tree), if mismatch show the icon/button.
It's the count only so there may be undetected changes, like rename or del+new.

That's one of the situations where I tried to improve Computer, or at least hack around its flaws. My design and UI decisions may not be the best, happy to receive proposals.

And after all, this is about the Computer library view, so if there are file / dir changes you'd need to rescan anyway. And, in my experience at least, significant file / directory modifications are very rare while playing.

[ronso0]

Btw #15571 or similar is required to make this feature work satisfactorily.

[ronso0]

Nice, I managed to also restore Computer bookmarks.
Depending on the tree level they are usually still not available after start, but are marked once the respective tree item is expanded.

Will update this branch soon.