ALAC/AAC/MP4: Fallback to ReplayGain tags from iTunes metadata atom

According to the ReplayGain specs (http://wiki.hydrogenaud.io/index.php?title=ReplayGain_1.0_specification, http://wiki.hydrogenaud.io/index.php?title=ReplayGain_2.0_specification), for MP4 the ReplayGain tags have to be written into an ID3 formatted ID32 atom.

I am unaware of any scanner that actually does that (please point me to one that does, if you know one).

Instead, software like Foobar and beaTunes write the tags into the iTunes metadata tag under "----":"replaygain_track_gain". Those tags can easily be viewed with software like AtomicParsley. But VLC seems to ignore those tags.

I'm assuming that VLC already parses the iTunes metadata atom and has easy access to any tags stored there. Therefore, even if it's not mentioned in the spec, it would be in the user's best interest, if VLC

First check the spec'ed location, i.e. the ID32 atom for any replay gain tags
If none are found, fall back to read the iTunes metadata tags
Display somewhere in the UI, what values are being used (separate ticket)

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