Add the ability to retarget a transporter without breaking and re-keying

[jeremyshannon]

Transporter pads cannot have their destinations changed, even if the destination no longer exists. You have to break the pad, wipe the key, then re-name it and re-link all other pads that point at it.
It should be possible to wipe the destination of a placed pad. Perhaps by clicking with the defunct destination key? That would prevent accidental erasure.

[jeremyshannon]

Closed by commit d50faba