Draw path to suspected destination if actual destination unknown

[dHannasch]

This allows an empire without Improved Space Scanner that sees a ship on two successive turns (in a situation where it's impossible to get mixed up with any other ship) to "walk it forward" to see where it's going to be in a few turns.

Note that while the displayed vector is always "correct" in the sense that the ship really is moving along that line, the displayed line segment always stops at the first star system that crosses its path, whereas the ship might in fact bypass that star system at warp. (It cannot know whether that star is the actual destination, because the empire doesn't know the actual destination.) I'm not too worried about that in terms of player confusion, but this does currently use the same PathSprite as known-for-sure paths. I mostly figure it's probably fine in the sense that it's not displaying anything false, the ship really is headed there, it just might not stop there.

This only touches Transport at the moment. This would obviously apply to ShipFleet as well, but before doing that I want to do more unification of Transport and ShipFleet to avoid adding yet more machinery duplicated between both, and that seems like it should be its own branch, since it makes major changes to two important classes but shouldn't change any behavior.

[BrokenRegistry]

Good Job, I like it! Still! I've made it optional to allow purist to disable it. By default, it's ON, to make it more visible. I'll let you replace "Basic Scanner" with a better naming, and phrase the description and the guide help. (In labels.txt)