Rev4 update

This is in sync with https://docs.google.com/document/d/1m7Cep2U5eCe5M62Y6a_4CNGF7QM4UgJtmMFq2FrRo3s/edit?usp=sharing

and implements the proposals there. Please carefully review (in particular removed messages). This will go soon in master after another community review round. Criticism now or never 8).
This high level command is in here intentional - it simplifies GCS development substantially, as a button can be linked directly to the command. It is then up to the vehicle to turn this into the on-vehicle instructions to make this happen.