gtk-demo: Fix the links example
gtk-demo: Fix the links example

The keynav dialog is transient to the example window; since the
example window is now modal, we need to make the keynav dialog
modal as well, so it can receive input.
Problem pointed out by Jasper St. Pierre.

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