Switch to Gtk.Fixed

[leolost2605]

Currently we animate the moving of launchers with basic css. This will cause issues because we pretty much don't have any control once animation has started and it doesn't work well once we get to implementing drop support from the applications menu. Therefore this PR switches to using a Gtk.Fixed and manually positioning as well as using Adw.TimedAnimation for the move animation of the launchers. It requires a bit more manual work from our side but can be a lot more powerful in the future since we have more control. Also since we have fixed dimension and we don't have any text in the launchers the biggest issues of using a Gtk.Fixed don't apply here.

Also this PR separates a LauncherManager extending the Gtk.Fixed from the MainWindow to keep the code base IMHO cleaner

@elementary/ux : Adw.TimedAnimation has a TON of easing modes and I've got no idea which one to use here so some input would be helpful :) (also maybe a different duration like 200 ms would be better?)

[vjr]

Place your bets! I bet on EASE_IN_OUT_QUAD being the one to go with, haha.

[leolost2605]

The last commits should fix most remaining issues and flickering when moving items which are also currently present in main. It also switches to 200ms animation duration and sets easing to EASE_IN_OUT_QUAD as suggested by @vjr which IMHO looks pretty good :)

[leolost2605]

TODO:

• [ ] right to left language support (is this required or is it enough to save the icons the other way round ?)
• [x] test on wayland