panels: Add X-GNOME-UsesNotifications to several desktop files
gnome-settings-daemon emits notifications related to these four control panels. If we add X-GNOME-UsesNotifications to their desktop files, the user will be able to control the notifications.
https://wiki.gnome.org/Initiatives/GnomeGoals/NotificationSource
Signed-off-by: Philip Withnall withnall@endlessm.com
See gnome-settings-daemon!63 (merged) for the other half of this.