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.
This reverts commit 30a1a79322.
This turns out to be unnecessary when you can set the titlebar
on the window and let GtkWindow handle the events. As a benefit,
we get the window menu on custom titlebars for free.
