gapplication: remove inactivity_timeout source on finalize
gapplication: remove inactivity_timeout source on finalize

The event source used to handle inactivity_timeout doesn't hold a
reference on the application. Therefore, it is possible for callback
function of the event source to run after the application has been
freed, leading to use-after-free problem. To avoid the problem, we
should remove the event source before the application is freed.

This should fix SIGBUS crash of gio/tests/gapplication on FreeBSD.
https://gitlab.gnome.org/GNOME/glib/issues/1846#note_566550

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib