Release an extra reference on GtkExpander:child

Release an extra reference on GtkExpander:child

The child of a GtkExpander is owned directly by the expander whenever
the "expanded" flag is unset.

We are adding an additional reference to the child of an expander when
expander is not expanded.

Additionally, if a GtkExpander is disposed while not expanded, we need
to explicitly release the reference on the child widget that we own.

This reference leak was masked in GTK3 by GtkContainer removing each
child from the parent container by recursively calling
gtk_widget_destroy().