GtkMenuButton: do not use gpointer for the arrow widget
GtkMenuButton: do not use gpointer for the arrow widget

Just a small cleanup, we can use a proper type.

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