gtk-demo: Add a way to launch individual demos
gtk-demo: Add a way to launch individual demos

Add a --run option which takes the name of an example and
launches it. Also add a --autoquit option which can be used
to quit after a given number of seconds.

davidgumberg/gtk,jigpu/gtk,davidgumberg/gtk,jadahl/gtk,davidgumberg/gtk,alexlarsson/gtk,jadahl/gtk,jigpu/gtk,jadahl/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jadahl/gtk,jadahl/gtk,jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,davidgumberg/gtk,grubersjoe/adwaita,davidgumberg/gtk,jigpu/gtk,jigpu/gtk,davidgumberg/gtk,alexlarsson/gtk,grubersjoe/adwaita,jadahl/gtk,jigpu/gtk,jigpu/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,alexlarsson/gtk,alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,alexlarsson/gtk,davidgumberg/gtk