#31 misc_support.cpp enhancement

[Martin Rohrbach]

I'm using a couple of additional helper-functions for quite
a while now. They are following the same pattern as the
ones in misc_support.cpp. So maybe they could be
usefule for others as well and be included in
misc_support.cpp.

ID3_AddBPM
ID3_RemoveBPMs
ID3_AddUserText
ID3_RemoveUserTexts

see misc_addon.cpp

[Martin Rohrbach]

code for described funcs