WIP: fix bug 436405: MetaData isn't updated while there is not metadata at all

Trying to fix https://bugs.kde.org/show_bug.cgi?id=436405

Contains two fixes:

• entry doesn't get inserted into tracks table when the track's metadata doesn't contain title. Fix: use track's name as title
• lyric doesn't get updated when the lyric is empty. Fix: update empty lyric but don't insert lyric entry to TrackMetaDataModel

Known issue:

• tracks that don't have metadata on playlist is marked invalid on second load (add it to playlist and exit then re-open, track is invalid)
• on fallback, using track file name as metadata's title isn't technically correct