XWM: fullscreen window fixes
This should two issues with Xwayland fullscreen windows:
- initially fullscreen window never appearing at all
- fullscreen window positioned such that part of the window title bar is visible
Hopefully this also fixes #114 (closed), but I never saw an issue with the panel. The panel is supposed to be on top of a fullscreen window anyway, when the fullscreen window is not active.
This MR is ordered after !338 (merged).