Add default hotkey to swap primary screens.

Also minor style changes

src/citra_qt/main.h

@@ -105,6 +105,7 @@ private slots:
     /// Called whenever a user selects the "File->Select Game List Root" menu item
     void OnMenuSelectGameListRoot();
     void OnMenuRecentFile();
+    void OnSwapScreens();
     void OnConfigure();
     void OnDisplayTitleBars(bool);
     void ToggleWindowMode();