GtkHeaderBar: Survive toggling custom / non-custom titles

GtkHeaderBar: Survive toggling custom / non-custom titles

The code was always adding a label widget as a child, but
then skipping over it in forall if a custom_title was present.
This confuses internal logic of GTK+ which assumes that it
can iterate over the entire widget hierarchy with forall,
to maintain state. Fix this by destroying the label when
a custom_title is set, and recreating it as needed.