Add option to easily create reverse of corridor

[Nate-Wessel]

We often draw a corridor one way then have to also manually add its reverse direction. It would be nice to have a little button that would automatically add a new corridor with those nodes reversed. Would want to check though that that doesn't already exist and remove the option for both corridors if it does.

Potential for added confusion where the reverse direction (if it even exists) is not the simple reverse of the nodes. E.g. Adelaide & Richmond.