Handle available screen rect calculation separately for X11 and Wayland
Back then, the panels on the same screen edge would not stack, meaning that we just had to take the tallest of them. Now they do stack, meaning we have to take the sum of the thickness of the panels on each side. This only affects Wayland, so X11 is preserved.
TO TEST: create multiple panels on the same side, open their settings, and make sure the setting panel do not intersect with any panel.
Edited by Niccolò Venerandi