reimplement MP audio mode 2 (active instance only)

src/frontend/qt_sdl/Window.h

@@ -131,6 +131,10 @@ public:
 
     void onAppStateChanged(Qt::ApplicationState state);
 
+    void onFocusIn();
+    void onFocusOut();
+    bool isFocused() { return focused; }
+
     void osdAddMessage(unsigned int color, const char* msg);
 
     // called when the MP interface is changed
@@ -264,6 +268,8 @@ private:
     int windowID;
     bool enabledSaved;
 
+    bool focused;
+
     EmuInstance* emuInstance;
     EmuThread* emuThread;