Fix album artist for FLAC/OGG files (with vorbis comments)

[Chocobozzz]

Fixes #3194

[ArnaudBienner]

Would be nice to also read back this in ParseOggTag.
Also, is this standard? (I don't remember if other fields we are setting in comment are standards, and if it is standard to do it this way. I'm just wondering).

Also, it might be worth to add some unit test for this (in song_test.cpp?)
Maybe it is a bit overkill. But this can also help you to test this is working well, and it's probably not long to do. As you prefer.

[Chocobozzz]

Yes good idea @ArnaudBienner for the tests I'll do it when I'll have time :)

ParseOggTag already implemented the ALBUM ARTIST field (line 470).

Infortunaly it doesn't seem that album artist field is a standard, but I always saw people use "ALBUMARTIST" or "ALBUM ARTIST" field name so I think it will be portable.

[Chocobozzz]

It's done :)