Sets user_data on the frame and the #GDestroyNotify that will be called when
the frame is freed. Allows to attach private data by the subclass to frames.
If a user_data was previously set, then the previous set notify will be called
before the user_data is replaced.
Sets user_data on the frame and the #GDestroyNotify that will be called when the frame is freed. Allows to attach private data by the subclass to frames.
If a user_data was previously set, then the previous set notify will be called before the user_data is replaced.