menuitem: Use .arrow style class for submenu indicators
menuitem: Use .arrow style class for submenu indicators

As requested by the Adwaita guys.

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