GtkMessageDialog: Allow action-area-border theming again
GtkMessageDialog: Allow action-area-border theming again

This border does not interfere with headerbar-vs-content placement,
and we can allow themes to change it without breaking dialogs.

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