listen to media key events of gnome settings daemon

* add shortcuthandler, which listens to the media key event
  provided by the gnome settings daemon via dbus
  (https://github.com/GNOME/gnome-settings-daemon/blob/master/plugins/media-keys/README.media-keys-API)
* fixes TWK-983
Merge pull request #189 from f1ori/gnome-media-keys

listen to media key events of gnome settings daemon