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. #1846 (comment 566550)