LibGUI: Let GModel specify the drag data type

GModel subclasses can now override drag_data_type() to specify which type
GAbstractView should set for drag data. The default implementation returns a
null string, which disables dragging from this widget.

Libraries/LibGUI/GSortingProxyModel.cpp

@@ -87,6 +87,11 @@ void GSortingProxyModel::update()
     target().update();
 }
 
+StringView GSortingProxyModel::drag_data_type() const
+{
+    return target().drag_data_type();
+}
+
 void GSortingProxyModel::set_key_column_and_sort_order(int column, GSortOrder sort_order)
 {
     if (column == m_key_column && sort_order == m_sort_order)