* Fixed pulleyindicator staying busy in collectionpage
* PlaybackBar now correctly disables next/previous buttons
* AppWindow now handles the pushing for pages when an items needs to be
visited.
* Clicking on an item of type audio now starts the audio player
Besides MPRIS support, this also adds support for hasPrevious() and
hasNext() in several parts to determine whether the
player/playlist/shuffler has a previous or next item.