simplify player model

This is an attempt to solve #136 (closed) without the neeeded core changes. The main idea is to create a new model for the player when set_playlist is called. Then it relies on add_song and remove_song to update the playlist when content changes. The only dynamic view right now is the playlistview. So it needs some changes.

The first three commits are harmless (well, they should be). The last one is the important one.

Update 4 march

• typos and minor changes
• use an enum for player model fields
• directly set _current_track_uri from load method of the player
• solve problems with drag and drop operations in playlistview