Fix Sloppy/mouse focus mode on Wayland
This MR:
- Moves the X11-only handling bits of sloppy/mouse focus mode into
src/x11
. - Makes crossing events generated by user input have a correct
clutter_event_get_source_device()
- Uses that to handle sloppy/mouse focus mode on Wayland only for real user input.
More particularly, this avoids unintended window focus after shell popups, closing the overview, etc.
Closes: #888 (closed)