Do not rely on Tracker generated metadata
Currently Music for songs without tags it expects Tracker to fill out the title metadata with the filename.
Better would be to extract the filename and use it if the title returns None, not relying on optional support in Tracker.
Currently Grilo has no direct support fir this (get_filename()), but for starters we could use get_url() mangling.
Related: tracker-miners#28 (closed)