Let the compositor size space filling panels
There were two issues preventing to let the compositor size such panels:
- the window needs to be anchored on opposite edges, so LayerShellQt requests size 0 (meaning the compositor should deciude) in that dimension
- QtWayland currently will not size windows smaller than the requested minimum size. We can work around this by having a minimum size that's actually smaller than the screen dimension. This fixes a horizontal and a vertical panel next to each other to no longer overlap. (together with layer-shell-qt!51 (merged)) CCBUG:440571