For most purposes this function is deprecated in favor of
g_object_set_data(). However, for historical reasons GTK+ stores
the GtkWidget that owns a GdkWindow as user data on the
GdkWindow. So, custom widget implementations should use
this function for that. If GTK+ receives an event for a GdkWindow,
and the user data for the window is non-NULL, GTK+ will assume the
user data is a GtkWidget, and forward the event to that widget.
For most purposes this function is deprecated in favor of g_object_set_data(). However, for historical reasons GTK+ stores the GtkWidget that owns a GdkWindow as user data on the GdkWindow. So, custom widget implementations should use this function for that. If GTK+ receives an event for a GdkWindow, and the user data for the window is non-NULL, GTK+ will assume the user data is a GtkWidget, and forward the event to that widget.