Finish making the fence (spline) tool

[sci-comp]

This feature should primarily be concerned with laying down a fence, and should not be expanded into a general purpose spline tool

A fence will be a fence, so we will be concerned about features like,

• alignment along the spline
• spacing (should each fence scene be assumed to have the same width, or do we need to determine bounds)
• jitter, a fence stands upright, but it may lean this way or that
• usability, this should be easier to use than placing segments manually

Edit: I haven't been able to figure out usability such that it's better than just placing fence pieces manually. This issue is stalled out for the time being!

[sci-comp]

I'll go ahead and close this since Path3D mode is implemented, but I don't entirely like the user experience for it. We can open another issue later to improve UX, when ideas are more clear.