[meta] Add option to FORCE X11 as Graphics Backend

* save the option on a external file because settings
  are loaded AFTER Qt window is created and then
  the graphics backend is already applied

Signed-off-by: Caio Oliveira <caiooliveirafarias0@gmail.com>

src/yuzu/main.h

@@ -411,6 +411,9 @@ private slots:
     void OnCreateHomeMenuApplicationMenuShortcut();
     void OnCaptureScreenshot();
     void OnCheckFirmwareDecryption();
+#ifdef __unix__
+    void OnCheckGraphicsBackend();
+#endif
     void OnLanguageChanged(const QString& locale);
     void OnMouseActivity();
     bool OnShutdownBegin();