The code for window overlays in the overview was quite a mess, clean it up and fix some annoying issues like the flashing of overlays if one is re-selected, the bug that windows can be closed if the close button isn't visible and the oversized window titles.
Also improve the keyboard navigation: Allow using the arrow keys immediately without pressing the down key and allow selecting the windows on a secondary screen after moving the pointer on one.
Also completely stop showing the overlay during animations of the window clone.
Depends on !346 (merged)