gtk-builder-tool: Validate templates
gtk-builder-tool: Validate templates

Catch an error that indicates the file looks like a template,
and then try again, this time with the template parsing API
of GtkBuilder. This is a little iffy, since we need to create
a 'fake' type and instance to pass in, but it works ok in
simple tests.

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