Don't rely on button allocations for invisible headers
Don't rely on button allocations for invisible headers

Now that we are not allocating treeview column buttons anymore
with invisible headers, we can't rely on their allocations for
other things like cell area computations anymore. Use x-offset
and width of the column directly, instead.

Lyude/gtk-,Lyude/gtk-,ahodesuka/gtk,jessevdk/gtk,bratsche/gtk-,chergert/gtk,davidt/gtk,Adamovskiy/gtk,ahodesuka/gtk,davidt/gtk,Distrotech/gtk2,Sidnioulz/SandboxGtk,Lyude/gtk-,jigpu/gtk,davidgumberg/gtk,jadahl/gtk,msteinert/gtk,jadahl/gtk,ahodesuka/gtk,jigpu/gtk,jigpu/gtk,chergert/gtk,alexlarsson/gtk,Distrotech/gtk2,jessevdk/gtk,davidgumberg/gtk,Adamovskiy/gtk,davidgumberg/gtk,jessevdk/gtk,Adamovskiy/gtk,alexlarsson/gtk,jadahl/gtk,jadahl/gtk,jadahl/gtk,Lyude/gtk-,jessevdk/gtk,davidt/gtk,chergert/gtk,ahodesuka/gtk,Distrotech/gtk2,ebassi/gtk,ebassi/gtk,davidt/gtk,Sidnioulz/SandboxGtk,bratsche/gtk-,grubersjoe/adwaita,Adamovskiy/gtk,msteinert/gtk,chergert/gtk,bratsche/gtk-,jigpu/gtk,Lyude/gtk-,ahodesuka/gtk,davidgumberg/gtk,ahodesuka/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,grubersjoe/adwaita,Sidnioulz/SandboxGtk,bratsche/gtk-,Sidnioulz/SandboxGtk,Lyude/gtk-,jessevdk/gtk,davidt/gtk,jadahl/gtk,Distrotech/gtk2,grubersjoe/adwaita,grubersjoe/adwaita,ebassi/gtk,davidgumberg/gtk,jigpu/gtk,ebassi/gtk,davidgumberg/gtk,alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,Adamovskiy/gtk,msteinert/gtk,davidt/gtk,ebassi/gtk,msteinert/gtk,Lyude/gtk-,Adamovskiy/gtk,grubersjoe/adwaita,jadahl/gtk,bratsche/gtk-,jadahl/gtk,Distrotech/gtk2,ebassi/gtk,Adamovskiy/gtk,Distrotech/gtk2,Adamovskiy/gtk,chergert/gtk,msteinert/gtk,alexlarsson/gtk,davidgumberg/gtk,alexlarsson/gtk,jessevdk/gtk,chergert/gtk,bratsche/gtk-,jigpu/gtk,Sidnioulz/SandboxGtk,chergert/gtk,alexlarsson/gtk,ahodesuka/gtk,grubersjoe/adwaita,msteinert/gtk,Sidnioulz/SandboxGtk,jessevdk/gtk,Lyude/gtk-,davidgumberg/gtk,chergert/gtk,ahodesuka/gtk