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.
THe idea behind the switching was to provide gapless playback, but it
made the code too complicated and to intertwined with QtMultiMedia.
I might attempt to implement it later, but I probably will put the code
in a separate class to make it easier to swap the QtMultiMedia
implementation with a multimedia implementation for a player with better
support for gappless playback.