scrolledwindow: Refactor scroll unit guessing into a separate function
scrolledwindow: Refactor scroll unit guessing into a separate function

Makes it clearer, and will be used in further places.

https://bugzilla.gnome.org/show_bug.cgi?id=749770

chergert/gtk,alexlarsson/gtk,jigpu/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,jadahl/gtk,Adamovskiy/gtk,jigpu/gtk,grubersjoe/adwaita,davidgumberg/gtk,Adamovskiy/gtk,chergert/gtk,davidgumberg/gtk,chergert/gtk,grubersjoe/adwaita,jigpu/gtk,davidgumberg/gtk,davidgumberg/gtk,jigpu/gtk,chergert/gtk,davidgumberg/gtk,Adamovskiy/gtk,jigpu/gtk,alexlarsson/gtk,chergert/gtk,jigpu/gtk,Adamovskiy/gtk,alexlarsson/gtk,jadahl/gtk,alexlarsson/gtk,jadahl/gtk,chergert/gtk,jadahl/gtk,Adamovskiy/gtk,davidgumberg/gtk,jigpu/gtk,chergert/gtk,chergert/gtk,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,Adamovskiy/gtk,grubersjoe/adwaita,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,davidgumberg/gtk,alexlarsson/gtk,jadahl/gtk,grubersjoe/adwaita,Adamovskiy/gtk,Adamovskiy/gtk,alexlarsson/gtk