gtk-builder-tool: Strip placeholders
gtk-builder-tool: Strip placeholders

glade likes to sprinkle <child><placeholder/></child> all
over the place - get rid of it.

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