The audio backend option automatically gets disabled when
emulation is running, so it's pointless to tell people what
would (not) happen if they changed the audio backend while
emulation is running.
Prior to this, ConfigMain.cpp was a large (52KB) cpp file that contained all of the UI setting code.
This breaks up the config code into subclasses of wxPanel, which are then just instantiated to add to the settings wxNoteBook. This keeps all the settings categories separated from one another and also cleans up the code in general.
