Connectable (wall, wire, etc.) editing

[gajop]

Create a way to edit things that can be linked.
For roads we might utilize texture painting (with layers), but for others it will probably include features.

[gajop]

I've a feeling this won't be trivial, but hey we can at least do it for Gravitas and maybe would be useful for a train game?

[gajop]

Removed road from this as it might be confused with directional map texture editing