Add a lifecycle method for manual theme item caching to Control

scene/gui/scroll_container.h

@@ -69,9 +69,14 @@ private:
 	int deadzone = 0;
 	bool follow_focus = false;
 
+	struct ThemeCache {
+		Ref<StyleBox> bg_style;
+	} theme_cache;
+
 	void _cancel_drag();
 
 protected:
+	virtual void _update_theme_item_cache() override;
 	Size2 get_minimum_size() const override;
 
 	void _gui_focus_changed(Control *p_control);