Add support for notifications

Add a NotificationManager object that watches the Player and
keeps a resident notification to control the application.

https://bugzilla.gnome.org/show_bug.cgi?id=702377
notification: Get current media directly from player
notification: Fix crash if artist or album is unknown
po: Add gnomemusic/notification.py to POTFILES.in
Runtime check for libnotify
Runtime check for libnotify