Implement favourites playlist along with starring tracks

'favorites' playlist added

favorites are starred in 'songs' and 'playlists' views.

(currently super hacky, storing favorite data as 'lyrics'. should update to storing in 'favourite' asap.)

add/remove favorite queries

view: make star renderer togglable

view: ugly solution to skip on_item_activated if we clicked on start cellrenderer

changed Tracker.SparqlConnection object to singleton (within TrackerWrapper)

prelim. toggle_favorite func (not yet called)

toggle favorite status in database when toggling star in Songs view

added togglable stars to playlists view

changed add/rm favorites to rely on song urls, not ids

https://bugzilla.gnome.org/show_bug.cgi?id=743901