configure_graphics: Reimplement bg_color

To specialized a setting to be worth adding to the shared_widget imo,
so add it roughly like before.

src/yuzu/configuration/configure_graphics.h

@@ -51,6 +51,8 @@ private:
     void changeEvent(QEvent* event) override;
     void RetranslateUI();
 
+    void Setup();
+
     void PopulateVSyncModeSelection();
     void UpdateBackgroundColorButton(QColor color);
     void UpdateAPILayout();