messageTray: Dispose Notification on destroy

messageTray: Dispose Notification on destroy

When the notification is destroyed we should also dispose the underneath GLib
object, and ensure that we don't dispose this twice.

In order to avoid this, don't destroy transient notifications that have been
already been removed and only destroy the resident notifications on activation
if they have not been destroyed earlier.
Thus connect after to the 'activated' signal and once the default handler has
been called destroy the notification if not requested earlier.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/559