GUI: Remove option to enable/disable native_mt32 via GUI or command-line

This patch is supposed to remove the option to enable or disable the native_mt32 flag in both the GUI and command line. Instead it sets native_mt32 based upon whether a device is selected in the MT-32 device popup. More details are provided with each included commit.

Also, a previous commit ("Update imuse_internal.h") found its way in here and I can't seem to remove it. It will fail if my previous pull request (Roland GS code clean-up) is accepted, as it already resides there.

Unfortunately, I cannot compile the code to test at present, so there could be some issues with the patch. Can someone please test for me? My apologies if this is a mess, but without being able to compile, it's rather difficult to see if I've missed anything.