Move tool and button to snap RoadPoints to terrain

[TheDuckCow]

I'm using terrain in a loose sense here to mean any mesh set up with collider. See this reference video in Unreal Engine at 8:46, a nice feature that lets you snap the RoadPoints (and presumedly the control points too) to the level of the terrain, lowering or increasing height of each one specifically. Right now we already do have the ability to click to add roadpoints onto colliders, but once placed there's no way to move to make sure they stay there. A function like this, or a more continual move+snap to terrain would be helpful.

This would be simialr but not quite as intensive of this other idea for shrinkwrapping the entire curve to colliders, since that operates on every loop cut of the road and not just the end RoadPoints: #144

This video also shows the mesh deform along terrain, which is a feature defined here #123 and continues to be one that I think long term will be very important for this project.