VisualBuilder: Don't allow moving/resizing widgets that are in a layout

Also paint these widgets' grabbers differently to make it stand out
visually which widgets have managed geometry. :^)

DevTools/VisualBuilder/VBWidget.h

@@ -72,6 +72,8 @@ public:
     Rect transform_origin_rect() const { return m_transform_origin_rect; }
     void capture_transform_origin_rect();
 
+    bool is_in_layout() const;
+
 private:
     VBWidget(VBWidgetType, VBForm&, VBWidget* parent);