LibGUI: Make GTableModel a retainable object.

It became clear that this class needs to support multiple owners.

LibGUI/GTableModel.h

@@ -34,7 +34,7 @@ private:
     GModelIndex m_index;
 };
 
-class GTableModel {
+class GTableModel : public Retainable<GTableModel> {
 public:
     struct ColumnMetadata {
         int preferred_width { 0 };