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.

alexlarsson/gtk,msteinert/gtk,ebassi/gtk,Distrotech/gtk2,jadahl/gtk,jigpu/gtk,Distrotech/gtk2,Lyude/gtk-,davidgumberg/gtk,chergert/gtk,Sidnioulz/SandboxGtk,jigpu/gtk,Distrotech/gtk2,bratsche/gtk-,chergert/gtk,msteinert/gtk,jigpu/gtk,davidgumberg/gtk,grubersjoe/adwaita,ahodesuka/gtk,Adamovskiy/gtk,ahodesuka/gtk,chergert/gtk,Lyude/gtk-,chergert/gtk,jadahl/gtk,ebassi/gtk,ahodesuka/gtk,jadahl/gtk,Adamovskiy/gtk,davidgumberg/gtk,jessevdk/gtk,grubersjoe/adwaita,Lyude/gtk-,chergert/gtk,msteinert/gtk,davidgumberg/gtk,Adamovskiy/gtk,msteinert/gtk,jigpu/gtk,msteinert/gtk,chergert/gtk,ahodesuka/gtk,jadahl/gtk,bratsche/gtk-,Lyude/gtk-,ahodesuka/gtk,jadahl/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,Distrotech/gtk2,bratsche/gtk-,alexlarsson/gtk,jigpu/gtk,ebassi/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,jessevdk/gtk,alexlarsson/gtk,jigpu/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,alexlarsson/gtk,jessevdk/gtk,alexlarsson/gtk,grubersjoe/adwaita,Adamovskiy/gtk,jadahl/gtk,grubersjoe/adwaita,Distrotech/gtk2,bratsche/gtk-,grubersjoe/adwaita,alexlarsson/gtk,ahodesuka/gtk,chergert/gtk,jigpu/gtk,chergert/gtk,jessevdk/gtk,jessevdk/gtk,Lyude/gtk-,Distrotech/gtk2,ahodesuka/gtk,Lyude/gtk-,ebassi/gtk,msteinert/gtk,Sidnioulz/SandboxGtk,Adamovskiy/gtk,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,bratsche/gtk-,ebassi/gtk,jigpu/gtk,Adamovskiy/gtk,ahodesuka/gtk,davidgumberg/gtk,bratsche/gtk-,jessevdk/gtk,Adamovskiy/gtk,jessevdk/gtk,ebassi/gtk,jadahl/gtk,davidgumberg/gtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,Adamovskiy/gtk,davidgumberg/gtk,Lyude/gtk-