Implemented Notification support using notify-send as opposed to libnotify. This is because notify_notification_send does a synchronous dbus call to org.freedesktop.Notification and typically results in DBus timeouts. In the near future we will rewrite this to just use our own proxy and do an async send.
Refactored our GVariant metadata generation for a KotoIndexedTrack into a dedicated koto_indexed_track_get_metadata_vardict function. This can then be used across our playback engine and MPRIS.
