treeview: Add back the Gdk pointer grab during column dragging
treeview: Add back the Gdk pointer grab during column dragging

The reparenting happening on the column header so it gets a movable
window breaks the implicit grab, so this is one situation were we
want a pointer grab, if just to replace it.

https://bugzilla.gnome.org/show_bug.cgi?id=732933

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