donotdisturb: switch x11 fullscreen detection to Xlib

[deltragon]

This is needed for Gtk4, as Gtk4 does not offer APIs to find/change other applications' windows. No functional change intended. This should keep the exact same behaviour on X11 as the Gtk3 implementation, as it sends the same X events.

This is best reviewed with whitespace ignored, as some of the logic stays the same (but changes indentation).

This should be the last blocker before #561 can be merged.