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
- [x] Show the unit being carried below the carryall (Needs good polish)
- [x] Base fly when idle (easy)
- [x] Code style and rules yaml

Later in other pr:
* Spawn outside screen and move into playing area after being build
* Transport units to the repair pad when clicked with the wrench
* Deliver a harvester when proc is build
* Show the upward pointing arrow on a selected carryable waiting to be picked up
* Fly more like a Dune2000 caryall rather than like a ra helicopter

Take a look at https://github.com/pchote/OpenRA/commit/6cb71660aaa5edc7d6665c93331c2ae248406b18