HackStudio: Reflect widget selections in the form widget tree view

You can now manipulate the widget selection either by clicking and
dragging the widgets using the cursor tool, or by interacting with
the form widget tree view. :^)

DevTools/HackStudio/WidgetTreeModel.h

@@ -15,6 +15,8 @@ public:
     virtual GModelIndex parent_index(const GModelIndex&) const override;
     virtual void update() override;
 
+    GModelIndex index_for_widget(GWidget&) const;
+
 private:
     explicit WidgetTreeModel(GWidget&);