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.cpp

@@ -75,3 +75,14 @@ void WidgetTreeModel::update()
 {
     did_update();
 }
+
+GModelIndex WidgetTreeModel::index_for_widget(GWidget& widget) const
+{
+    int parent_child_index = 0;
+    for (auto& parent_child : widget.parent_widget()->child_widgets()) {
+        if (parent_child == &widget)
+            return create_index(parent_child_index, 0, &widget);
+        ++parent_child_index;
+    }
+    return {};
+}