Merge pull request #188 from f1ori/master

 listen to media key events provided by gnome settings daemon
Add the possibility for JSResolvers to specify that a given URL is checked/valid.
Revert "listen to media key events of gnome settings daemon"

This reverts commit 966ab37b94527cdcd8c1b1fea8984f68f59a2be4.
Add desktop notifications for received tracks
Automatic merge of Transifex translations
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)