plugins: Add desktop-entry hints to notifications
This allows the notifications to be tied back to an application (or, in this case, control panel capplet) which is related to it; and allows the user to specify per-app preferences for how the notifications are to be displayed.
https://wiki.gnome.org/Initiatives/GnomeGoals/NotificationSource
Signed-off-by: Philip Withnall withnall@endlessm.com
This supersedes !36 (closed).