The following patch implements toggle-able stars in the Songs and Playlists views to mark a song as favorite/non. Favorite songs are automatically added to the Favorite Songs playlist. Favorite status is recorded in tracker by means of the `nao:hasTag nao:predefined-tag-favorite` property.

ISSUES TO BE RESOLVED: [MediaAudio object].get/set_favourite() methods are currently not working. I've hacked around this by storing the favorite value in [MediaAudio object].get/set_lyrics() (an empty string -- "" -- is False, any non-empty string value is True). (Note that this does not store any data in nmm:lyrics or similar in tracker.) Obviously this vastly un-ideal and hard to read and should be fixed as soon as we can get get/set_favourite() working.