Introduce a cache manager
Cachemanager makes sure that for each unique spotify object only 1
Qspotifyobject exists.
This commit also tries to reduce memory consumption of QSpotifyTrack

Thanks to xplodwild from #spotify irc for the idea
Comment out not working toplistBrowse
Add new RingBuffer to reduce memory usage while playing
Previously we filled a QByteArray until the track was ended, this could
lead to very high memory consumption while playing long tracks.
Fix inbox and starred tracklist order
Allow to switch playqueue tracklist while playing