Add generic support for reading writing Replaygain fields

[IJabz]

From https://java.net/jira/browse/JAUDIOTAGGER-446

I could not find on the documenation how to read Replaygain ID3 tags: REPLAYGAIN_TRACK_GAIN REPLAYGAIN_TRACK_PEAK REPLAYGAIN_ALBUM_GAIN REPLAYGAIN_ALBUM_PEAK

Is this supported ? If so, how can I read those ? Alternatively, mp3gain can also (default behavior) store ReplayGain information in APE tags on MP3. Is it possible to read those APE Replaygain tags and if so how ?