citra_qt/configuration: Add background color config

This is actually very easy to do. I added a label and a push button for it, and used the Qt standard dialog QColorDialog for picking colors.

src/citra_qt/configuration/configure_graphics.h

@@ -26,4 +26,5 @@ private:
 
 private:
     std::unique_ptr<Ui::ConfigureGraphics> ui;
+    QColor bg_color;
 };