FIX wrong played track

Previously, the track played did not correspond to the displayed title and
artist (which were correct).
This seems to fix also the "Attempt to call getDuration without a valid
mediaplayer" bug.

Additional changes:
* MusicItem.local_id is now long (as in sqlite)
* MusicItem.play_order is now long (as in sqlite)
Merge branch 'dev/solo'