Improve the editor export template downloader

[Calinou]

• Begin downloading as soon as the list of mirrors is fetched if only one mirror is available.
• Only show the help text if there's more than one mirror available.
• Only show the progress bar once the download has started.
• Hide the list of mirrors and the help text once the download has started.
• Make the window slightly shorter due to the low number of mirrors available.

PS: To test this locally, you need to change version.py's status line to rc3 then recompile.

[YuriSizov]

There are a couple of unresolved issues here still.

1. If there are no sources, the UI just ends up in a weird state. An error is shown in a third (!!!) popup and then, when you close it, the second one remains with a fictitious progress:

2. You can close the second popup as the templates download, but it keeps downloading. Attempts to bring that popup back result in a flash of progress and then an error that the archive couldn't be fetched. However, it still keeps downloading, and in flashes you can see it progressing, and eventually the UI properly updates with the installed version as it finishes.

3. In the old version you couldn't Shift-Click to open the mirror in the browser after you start downloading, despite the link still showing the pointing finger cursor. In the new version you can't open the link in the browser at all if there is only one mirror. If this is a desired feature, then the URL needs to be displayed still somewhere, and the Shift-Click as it downloads needs to be fixed.

I'm not entirely sure why we need 2 popups here. We can make do with a single one and a better representation of installed versions and mirrors with a tree control.

[Calinou]

In the old version you couldn't Shift-Click to open the mirror in the browser after you start downloading, despite the link still showing the pointing finger cursor. In the new version you can't open the link in the browser at all if there is only one mirror. If this is a desired feature, then the URL needs to be displayed still somewhere, and the Shift-Click as it downloads needs to be fixed.

The Shift-clicking part was mainly added for people having issues with slow downloads due to the low HTTPClient buffer size, but this is now solved since 3.3. It could be removed eventually.

[YuriSizov]

Here's a little mockup of how this unified window may look:

2021-05-16_03-44-50.mp4

1. Current version at the top, shows either the missing status, or the path to the downloaded templates. Progress is also displayed there, inline. While the progress is active Close button at the bottom shows a warning before closing. Closing the window should abort the download.
2. Download button next to the current version automatically refreshes the sources and then uses the first one to download the templates. At the same time the list can be manually refreshed and the user can click on the Download button next to a particular source they want. We can also refresh the list of sources automatically when the popup opens if the current version is missing.
3. We keep the button to open the source in the browser, can still be helpful for people behind some corporate proxy settings or something like that.
4. Both current version and the list of all installed versions have a way to open the folder in the filesystem to access the files if needs be.
5. All warnings, errors, confirmations are shown as a second level popup, as usual.

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Sorry for a weird look in the video, I made it as a project, not as an editor change.

Edit: I also forgot the install from file button, but it should be easy to find a place for it.

[YuriSizov]

I've opened #48902 to replace this one.

[Calinou]

Closing, since this is superseded by #48902.