Implement true music driver selection from the GUI options dialog.
This implementation depends on a new, transient config domain called
"user-overrides". Probably any other config changes made from the
options dialog will need to go into this domain, which is now
loaded after the game settings are loaded.
I hope this is the right way to do it. I'm still a little fuzzy
about our game settings architecture. If anybody suddenly gets
a "[user-overrides]" section showing up in their scummvm.ini or
scummvm.rc files, you'll know why.
svn-id: r8434
c117ef1e50