don't process hold note logic while paused

SM_Pause can be sent from elsewhere to pause
START unpauses when paused
don't move on to another screen when paused

stepmania/src/ScreenMessage.h

@@ -16,6 +16,7 @@ enum ScreenMessage {
 	SM_GainFocus,
 	SM_LoseFocus,
 	SM_StopMusic,
+	SM_Pause,
 	SM_User	= 100
 };