ADD directional arrows to highway=*, waterway=*, railway=*, aerial_way=* when selected.

ADD directional arrows to highway=*, waterway=*, railway=*, aerial_way=* when selected.

This was inspired by http://trac.openstreetmap.org/ticket/3380. I actually think it works pretty well. Even on my ancient desktop, where potlatch as a whole runs pretty slow, I don't see any performance hit. I've tried to keep it as unobtrusive as possible - in many instances you need to zoom in to see the arrows, which I think is ok.