Take Dan's advice and put the replay gain tag reading code somewhere that means Amarok itself can use it (so we can use it in Meta::File).

Note that the new header and cpp file are because amarokcollectionscanner doesn't link against amaroklib, and so can't just include MetaUtility.h (MetaUtility.cpp uses amaroklib stuff).

Of course, this should all go into taglib eventually, probably TagLib 2 when that happens.