meta_screen_get_mouse_window: change annotation to allow parameter not_this_one to be null

This is part of the fix for for Cinnamon issue #1945
Merge pull request #78 from autarkper/get-mouse-window-allow-none

meta_screen_get_mouse_window: change annotation to allow parameter not_this_one to be null
1.7.5
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
1.8.0
Merge remote-tracking branch 'upstream/master'