LibGUI: Allow rollback of model editing delegate input

In the StringModelEditingDelegate convenience class, we simply hook up
the escape key to editor rollback. This means you can cancel an ongoing
cell edit by pressing escape. :^)

Libraries/LibGUI/AbstractView.cpp

@@ -147,6 +147,10 @@ void AbstractView::begin_editing(const ModelIndex& index)
         model()->set_data(m_edit_index, m_editing_delegate->value());
         stop_editing();
     };
+    m_editing_delegate->on_rollback = [this] {
+        ASSERT(model());
+        stop_editing();
+    };
 }
 
 void AbstractView::stop_editing()