libdoomsday|DED: Added defn::Music
Refactor|libdoomsday|DED: Store Music definitions in a DEDRegister

Also, Music definitions may now be copied.
API: Access DED Music definitions in the database directly

The old Def_Get/Def_Set API mechanism for manipulating the music defs
was removed as one can now access the music def database directly.

Plus some minor cleanup.