Add hotkey to toggle screen emphasis

Toggles which screen is emphasized when the screen sizing is set to
emphasize top or bottom. (Closes issue #1565)

src/frontend/qt_sdl/main.h

@@ -85,6 +85,7 @@ signals:
     void windowFullscreenToggle();
 
     void swapScreensToggle();
+    void screenEmphasisToggle();
 
 private:
     void drawScreenGL();
@@ -331,6 +332,7 @@ private slots:
     void onUpdateVideoSettings(bool glchange);
 
     void onFullscreenToggled();
+    void onScreenEmphasisToggled();
 
 private:
     void closeEvent(QCloseEvent* event);