HackStudio: Start implementing basic widget selection in CursorTool

You can now select widgets by clicking on them with the CursorTool,
and toggle the selection state of a widget by Ctrl+clicking it.

DevTools/HackStudio/FormEditorWidget.h

@@ -18,12 +18,61 @@ public:
 
     void set_tool(NonnullOwnPtr<Tool>);
 
+    class WidgetSelection {
+    public:
+        bool is_empty() const
+        {
+            return m_widgets.is_empty();
+        }
+
+        bool contains(GWidget& widget) const
+        {
+            return m_widgets.contains(&widget);
+        }
+
+        void toggle(GWidget& widget)
+        {
+            if (contains(widget))
+                remove(widget);
+            else
+                add(widget);
+        }
+
+        void set(GWidget& widget)
+        {
+            clear();
+            add(widget);
+        }
+
+        void remove(GWidget& widget)
+        {
+            ASSERT(m_widgets.contains(&widget));
+            m_widgets.remove(&widget);
+        }
+
+        void add(GWidget& widget)
+        {
+            m_widgets.set(&widget);
+        }
+
+        void clear()
+        {
+            m_widgets.clear();
+        }
+
+        WidgetSelection() {}
+    private:
+        HashTable<GWidget*> m_widgets;
+    };
+
+    WidgetSelection& selection() { return m_selection; }
+
 private:
     virtual void paint_event(GPaintEvent&) override;
 
     explicit FormEditorWidget(GWidget* parent);
 
     RefPtr<FormWidget> m_form_widget;
-
     NonnullOwnPtr<Tool> m_tool;
+    WidgetSelection m_selection;
 };