New: Allow coupling

[leezer3]

This PR allows us to couple to loose cars that we've previously dropped.

Currently, driving into a train with both at less than the critical collision speed (this is a fixed 8km/h value) will couple us.

Heavy WIP, but now appears to be working correctly.

TODO:

• Trains need a way of distinguishing what they are. TFOs have this already by virtue of being a different type, but it's not quite right.
• Need to implement a 'do you wish to couple' yes / no dialog.
• It would be useful to allow us to place loose cars independantly on the route (sidings etc) so we could pick them up - Cars from the train.xml format would seem to fit this bill, as they should contain most if not all of the properties required. Would also require a new routefile command (Single XML, containing all loose cars??) Possibly for a later PR.

Glitches:

• ~~Beacon reciever will likely be broken.~~
• ~~Seem to need to switch back to exterior view again to get our coupled car to show.~~
• ~~It's possible to couple with a preceeding train stopped at a station. This shouldn't happen.~~
• ~~We should remove a disposed train from the list. Otherwise coupling / decoupling repeatedly causes the train list to grow exponentionally. Check if we rely on the length of this elsewhere???~~ Improved the logic, but this needs more testing.