Resolve deadlock on background change.

Due to how glib handles file monitors in a separate thread, unreferencing a monitor without canceling first can cause a race and deadlock.

For more details, refer to upstream issue: https://gitlab.gnome.org/GNOME/glib/-/issues/1941

Fixes #188 (closed)

Signed-off-by: Dan Merillat git@dan.eginity.com