HackStudio: Show the edited form widget's widget tree in the tree view

This patch introduces a simple WidgetTreeModel that models the widget
tree inside of a given root GWidget.

DevTools/HackStudio/WidgetTreeModel.cpp

@@ -0,0 +1,77 @@
+#include "WidgetTreeModel.h"
+#include <AK/StringBuilder.h>
+#include <LibGUI/GWidget.h>
+#include <stdio.h>
+
+WidgetTreeModel::WidgetTreeModel(GWidget& root)
+    : m_root(root)
+{
+    m_widget_icon.set_bitmap_for_size(16, GraphicsBitmap::load_from_file("/res/icons/16x16/inspector-object.png"));
+}
+
+WidgetTreeModel::~WidgetTreeModel()
+{
+}
+
+GModelIndex WidgetTreeModel::index(int row, int column, const GModelIndex& parent) const
+{
+    if (!parent.is_valid()) {
+        return create_index(row, column, m_root.ptr());
+    }
+    auto& parent_node = *static_cast<GWidget*>(parent.internal_data());
+    return create_index(row, column, parent_node.child_widgets().at(row));
+}
+
+GModelIndex WidgetTreeModel::parent_index(const GModelIndex& index) const
+{
+    if (!index.is_valid())
+        return {};
+    auto& widget = *static_cast<GWidget*>(index.internal_data());
+    if (&widget == m_root.ptr())
+        return {};
+
+    if (widget.parent_widget() == m_root.ptr())
+        return create_index(0, 0, m_root.ptr());
+
+    // Walk the grandparent's children to find the index of widget's parent in its parent.
+    // (This is needed to produce the row number of the GModelIndex corresponding to widget's parent.)
+    int grandparent_child_index = 0;
+    for (auto& grandparent_child : widget.parent_widget()->parent_widget()->child_widgets()) {
+        if (grandparent_child == widget.parent_widget())
+            return create_index(grandparent_child_index, 0, widget.parent_widget());
+        ++grandparent_child_index;
+    }
+
+    ASSERT_NOT_REACHED();
+    return {};
+}
+
+int WidgetTreeModel::row_count(const GModelIndex& index) const
+{
+    if (!index.is_valid())
+        return 1;
+    auto& widget = *static_cast<GWidget*>(index.internal_data());
+    return widget.child_widgets().size();
+}
+
+int WidgetTreeModel::column_count(const GModelIndex&) const
+{
+    return 1;
+}
+
+GVariant WidgetTreeModel::data(const GModelIndex& index, Role role) const
+{
+    auto* widget = static_cast<GWidget*>(index.internal_data());
+    if (role == Role::Icon) {
+        return m_widget_icon;
+    }
+    if (role == Role::Display) {
+        return String::format("%s (%s)", widget->class_name(), widget->relative_rect().to_string().characters());
+    }
+    return {};
+}
+
+void WidgetTreeModel::update()
+{
+    did_update();
+}