gtkaboutdialog: support showing buttons in action area
gtkaboutdialog: support showing buttons in action area

Give GtkAboutDialog buttons in the action area instead of hard-coding
use-header-bar to TRUE. This is for environments which don't have the
Gtk/DialogsUseHeader xsetting turned on.

https://bugzilla.gnome.org/show_bug.cgi?id=730893

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