Gtk.Bin widget is a container with just one child.
It is not very useful itself, but it is useful for deriving subclasses,
since it provides common code needed for handling a single child widget.
Many GTK+ widgets are subclasses of
|Returns:||the child of self, or
None if it does
not have a child.
Gets the child of the
None if the bin contains
no child widget. The returned widget does not have a reference
added, so you do not need to unref it.