* Moved stylesheet methods into TomahawkStyle.
* Use TomahawkStyle's methods to apply stylesheets.
* Removed obsolete png.
* Emit a signal when the currentTrackPlaylist changes in AudioEngine.
* Implemented Flexible(Tree)View's isBeingPlayed().
* Style fixes in SourceTreeView.
* Style fixes in SourcesModel.
* Style fixes in GenericPageItems.
* Implemented LovedTracksItem's isBeingPlayed().
* When a track starts playing, check if we need to update the sidebar's speaker-icon.