DSi power button and volume switch support (#1630)

* Add proper BPTWL interrupts

* Added DSi power button and volume switch hotkeys

* Added hardware reset workaround

* Adjusted syntax to follow guidelines

* Added DSi output volume synchronization

* Fix trivial member function error

src/frontend/qt_sdl/main.h

@@ -87,6 +87,8 @@ signals:
     void swapScreensToggle();
     void screenEmphasisToggle();
 
+    void syncVolumeLevel();
+
 private:
     void drawScreenGL();
     void initOpenGL();