Multiple accounts support

[KitsuneRal]

This adds (raw) support for multiple accounts. It basically has 3 parts:

• the first two commits are cleanup, logging tweaks, update to the latest lib API, refactoring. This can be (carefully) disposed of if it has something wrong. The latest lib (from the not yet merged Code tweaks and cleanup libQuotient#62) is necessary to expose Connection from Room.
• the next two commits are preparation. Here I unbind components from "global" Connection and make some code more defensive so that I don't need to take care of deleted connections too much in MainWindow.
• the last commit is the main thing. Here I replace a single Connection pointer with a container, rewire connections adding/dropping and modify RoomListModel so that it can show rooms from several accounts as well as delete rooms from no more available connections.

The provided UI and functionality are as follows:

• A user can login to several accounts at the same time (Login menu item is always enabled and there are multiple Logout menu items, by the number of accounts)
• Most of the UI components now deal with room's connections rather than a single global connection thrown around through parameters and setConnection() calls.
• Room list shows all rooms from all accounts. In practice the rooms are grouped by account, in the order of the first sync response. This needs improvement desperately but is workable for smaller cases.

If you just want to try/review multiple accounts feature and do not plan to further contribute, you can stop reading at this point.

Things to do/discuss further with libqmatrixclient and Q around this functionality (and adventurous minds can already fork this branch and make a "PR to PR"; discussions in #quaternion are also more than welcome):

1. There's a dubious distinction between loginError and loggedOut signals in Connection. In reality loginError is emitted in cases where something terribly bad goes on with user's access - such as AuthenticationRequired response returned from a /sync REST call. I suppose we have to merge these into one signal.
2. A logged out connection lives for a very short time and its status in general is unclear. In theory the libqmatrixclient code could support relogin in such case (and there's a reconnect() method that does exactly that) - however, Quaternion simply disposes of logged out connections as soon as it detects them. Given that logout (as well as login error, see the previous question) basically mean that the password is not (or no more) effectual, seems that this is the only reasonable way to deal with such connections. If that's the case, emitting Connection::loggedOut may be followed by deleteLater() right in the Connection code.
3. There's quite a bit of connection orchestration code that now resides in MainWindow but is very well worth putting in the library. There's a rationale for a QMatrixClient::ConnectionManager class. A similar RoomManager class would also be very nice to have and could serve as a library base for the room list model, or even the model itself.
4. RoomListModel, apparently needs to be converted to a tree. This is needed at least to select them by account, and also to make use of room tags. There's quite a bit of interesting work on the backend here; one will have to master QAbstractTreeModel.
5. You cannot do anything but login/logout with accounts. Given that we don't even have account_data parsed from sync(), this makes sense so far, but in the future account settings should be extended with more centralised login experience (see the most recent GNOME for a possible example, and KDE, I guess, has a similar thing), editing per-account things like personal userpic/avatar and "remember me" flag, etc. A big piece of work, mostly on the UI.

[AppVeyorBot]

✅ Build Quaternion 0.0.263 completed (commit 97937237c0 by @KitsuneRal)

[AppVeyorBot]

✅ Build Quaternion 0.0.264 completed (commit b47f0c715e by @KitsuneRal)