gtk3-demo: Fix two issues with the builder demo
gtk3-demo: Fix two issues with the builder demo

The ui file was generating a warning about AtkAction not
being implemented, and the about dialog had a missing icon.

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