add _gtk_widget_get_aux_info_or_defaults()
add _gtk_widget_get_aux_info_or_defaults()

This is better than peeking aux info then testing != NULL
in several ways:
- it returns const aux info so if we don't create we can't write
- it ensures that the default we assume if aux_info is NULL is
  the same as the default we set if we've created the aux info
- it avoids typing in != NULL checks

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