D2K: Implement automatic Carryall

This is a big WIP, the basic is working and wanted to share this with you guys. Please do not merge yet.

Its buildable from a heavy factory and is transporting harvs to and from procs. For this I've added some hooks into the harvester code.

Before merge i've got this todo list, in my order of preference:
- [x] Carry harvesters to and from spice fields
- [ ] Show the unit being carried below the carryall (Already got a hint from pchote on irc to look at the TD transports)
- [ ] Base fly when idle (easy)
- [ ] Transport units to the repair pad when clicked with the wrench (where's that base repair code?)
- [ ] Spawn outside screen and move into playing area after being build (no clue yet)
- [ ] Code style and rules yaml

Maybe later:
- [ ] Deliver a harvester when proc is build
- [ ] Fly more like a Dune2000 caryall rather than like a ra helicopter
- [ ] A (non buildable) ra chinook carying vehicles (for fun)

I'd like some feedback, about the global direction i've taken with the code, and need some help to implement the other goals in the todo list.
```csharp
var p = carryable.Trait<IPositionable>();
```

according to our https://github.com/OpenRA/OpenRA/wiki/Coding-Standard