Allow for notifications to have actions on the lockscreen

This is implemented using desktop file key X-Phosh-Lockscreen-Actions that allow to list action prefixes. Those prefixes are allowed as actions on the lock screen and the user can invoke them without unlocking. If such an action is present the notification body of the correspoinding will also be shown.

There currently are no provisions for allowing actions that do require unlock.

E.g. to allow the user to stop alarms and snooze in gnome-clocks we added

  X-Phosh-Lockscreen-Actions=app.stop-alarm::;app.snooze-alarm::;

to

  /usr/share/applications/org.gnome.clocks.desktop

Closes: #620 (closed)

Signed-off-by: Guido Günther guido.gunther@puri.sm