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>