GtkHeaderBar: Make the position child property writable
GtkHeaderBar: Make the position child property writable

Glade asssumes that it is, and without this, rearranging children
in glade is very cumbersome.

grubersjoe/adwaita,jigpu/gtk,chergert/gtk,Lyude/gtk-,jigpu/gtk,jadahl/gtk,jadahl/gtk,chergert/gtk,Adamovskiy/gtk,Lyude/gtk-,alexlarsson/gtk,jigpu/gtk,jadahl/gtk,grubersjoe/adwaita,Adamovskiy/gtk,Adamovskiy/gtk,jadahl/gtk,davidgumberg/gtk,chergert/gtk,alexlarsson/gtk,Lyude/gtk-,jigpu/gtk,davidgumberg/gtk,jadahl/gtk,ahodesuka/gtk,ahodesuka/gtk,jadahl/gtk,alexlarsson/gtk,chergert/gtk,Adamovskiy/gtk,davidgumberg/gtk,ahodesuka/gtk,jigpu/gtk,grubersjoe/adwaita,grubersjoe/adwaita,ahodesuka/gtk,Adamovskiy/gtk,alexlarsson/gtk,Lyude/gtk-,davidgumberg/gtk,chergert/gtk,alexlarsson/gtk,chergert/gtk,davidgumberg/gtk,alexlarsson/gtk,Lyude/gtk-,grubersjoe/adwaita,grubersjoe/adwaita,Lyude/gtk-,alexlarsson/gtk,ahodesuka/gtk,Adamovskiy/gtk,ahodesuka/gtk,davidgumberg/gtk,grubersjoe/adwaita,ahodesuka/gtk,chergert/gtk,jigpu/gtk,Adamovskiy/gtk,jigpu/gtk,Adamovskiy/gtk,ahodesuka/gtk,chergert/gtk,grubersjoe/adwaita,davidgumberg/gtk,jigpu/gtk,jadahl/gtk,Lyude/gtk-,Lyude/gtk-,davidgumberg/gtk,alexlarsson/gtk,jadahl/gtk