GtkCssProvider: Factor out a function
GtkCssProvider: Factor out a function

Move finding the gtk.css file into its own function. We will
make this more complicated in the near future.

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