The problem here is that our ConfigDialog uses "" as domain name,
this will result in the assert in common/config-manager.cpp:323
getting triggered, when using ConfMan.removeKey.
Since setting the domain to the game's domain does not seem save
according to the FIXME in gui/dialogs.cpp about this domain hackery,
I decided to adapt the OptionsDialog to only call ConfMan.removeKey
for "music_driver" etc., when the music driver widgets are present.
This is consistent with the over uses of removeKey in
OptionsDialog::close too.
svn-id: r50468