Drawing line as an arrow

[WaDelma]

I started to convert an adhoc UI thing that I was using to conrod. I found that there is Graph widget which is just what I wanted, but my graph is dag which means I want to draw edges as arrows which doesn't seem to be supported by conrod right now.

To support this new variant Arrow would need to be added to the Cap enum and there needs to be two different caps for each line.