Minor improvement for AlbumInfo
- Use the AlbumInfo check in the cover manager.
- Simplify getKey()
Improve cover request robustness : Let the cover manager triggers the cover view tag with the album key.
Avoid the use of a different album key to tag the view (in case of album key mismatch the cover notifications are ignored by the cover listeners)
Revert "Improve cover request robustness : Let the cover manager triggers the cover view tag with the album key."

This reverts commit 9555f3ecd51d9d80d49e0cf959668d13e9e4caa8.
Merge branch 'master' of https://github.com/abarisain/dmix into newmaster
make albums unique in list