qt: support CSD shadows in compositor_wayland

This was supported in compositor_x11 via _GTK_FRAME_EXTENTS, with Wayland we can make use of custom margins for the shadows as I do here. I have also tried to use xdg-shell's custom window geometry setting, but I realized that it was not compatible with Qt Wayland window and surface as it changed the size of the window and surface as well.

Qt already supports Adwaita style decoration that contains shadows, but it is disabled if FramelessWindowHint is used or the desktop environment is not gnome.

Note that this requires Qt 6.5. It is also possible to do this with earlier Qt versions but that requires presence of private Qt Wayland headers and I did not want to bother with it.

Request review @chub.