[Media] Improve tag extraction

* Prefer id3v2 (if any)
* Show album and artist as 'Unknown' if it's not found
* Show filename as title if there is no title

addresses
 https://github.com/owncloud/apps/issues/133
https://github.com/owncloud/apps/issues/445

might also be helpful for
https://github.com/owncloud/apps/issues/131
https://github.com/owncloud/apps/issues/566

cc @icewind1991