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>
11 jobs for power-notifications in 4 minutes and 48 seconds (queued for 3 minutes and 23 seconds)
Status Job ID Name Coverage
  Build
passed #137182
build

00:01:55

 
  Test
passed #137184
asan

00:01:59

passed #137183
test

00:00:53

 
  Manual
manual #137189
allowed to fail manual
armv8
manual #137185
allowed to fail manual
flatpak
manual #137188
allowed to fail manual
msan
manual #137190
allowed to fail manual
ppc64le
manual #137186
allowed to fail manual
tsan
manual #137187
allowed to fail manual
ubsan
 
  Review
manual #137191
allowed to fail manual
review
manual #137192
allowed to fail manual
stop_review