GtkDialog: Support adding action widgets in ui files
GtkDialog: Support adding action widgets in ui files

GtkDialog has convenience API for adding action widgets that are
either placed in the action area or the headerbar. This commit
makes the same functionality available from GtkBuilder ui files
by specifying "action" as the child type.

https://bugzilla.gnome.org/show_bug.cgi?id=728846

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