improvements to sysDef and UX changes

- "no undo" in clear option
- only display instrument types depending on current systems

src/engine/dispatchContainer.cpp

@@ -44,6 +44,7 @@
 #include "platform/qsound.h"
 #include "platform/vera.h"
 #include "platform/x1_010.h"
+#include "platform/su.h"
 #include "platform/swan.h"
 #include "platform/lynx.h"
 #include "platform/bubsyswsg.h"
@@ -311,6 +312,9 @@ void DivDispatchContainer::init(DivSystem sys, DivEngine* eng, int chanCount, do
     case DIV_SYSTEM_MMC5:
       dispatch=new DivPlatformMMC5;
       break;
+    case DIV_SYSTEM_SOUND_UNIT:
+      dispatch=new DivPlatformSoundUnit;
+      break;
     case DIV_SYSTEM_DUMMY:
       dispatch=new DivPlatformDummy;
       break;