Allow rail signal/chain signal placement on ghost rails

[Chibiabos]

Yes, I know you can include signal/chain signals in blueprints, but often I have to dynamically link rails using the rail planner to hook up to new ore patches to harvest or navigate trains around water or other obstacles, etc. Its a bit annoyingly tedious that I either have to wait for bots to actually place the rails before I can place signals/chain signals or their blueprints or go do something else and try to remember to come back to place them.

It would be a lot simpler if I could simply place them even while there is just a ghost awaiting construction bots so I don't have to actually wait for the construction bots.