[Tracker] Ontology change: Add more writeback to NMM

[Adrien Bustany <abustany gnome org>]

Hi, 

I'd like to request the addition of writeback to the two following
properties:
- nmm:artist
- nmm:musicAlbum

Currently, if you change the artist or album of a song in tracker, changes
cannot be writeback'ed... The writeback of these properties requires
additional
query (to fetch the name of the album/artist name), but I think it's not
much
of a performance concern.

Cheers

Adrien