load replaygain tags on mp4 style files
load tags when opening file so they are available when we try to read replaygain

provide a fallback file type to tagloader based on content

pass content type to DVDplayer for types that support tags, in order to set a fallback filetype
Merge pull request #3587 from Nachtzuster/ma4_replaygain

fix: Use replaygain tags for .m4a type family files too, closes Ticket #13948