public List<unowned Window> get_children_with_user_data (void* user_data)
Gets the list of children of this known to GDK with a particular
user_data set on it.
The returned list must be freed, but the elements in the list need not be.
The list is returned in (relative) stacking order, i.e. the lowest window is first.
user data to look for
list of child windows inside this