Add DIGIBooster Pro modules to list of known extensions

I have a collection of .dbm files (DIGIBooster Pro) tracker from Amiga :)
They are properly handled by libmodplug but was not visible in xbmc music files. This patch fixes it. To see and play, only the change in m_musicExtensions was necessary, but i also added a mime types. Don't know for sure if it is ok, but i hope so :)
Do i need to add it somewhere else (eg TagLoaderTagLib.cpp)?

Please review