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 line needs some discussion / thought. We really should make it mavlink_param_value_double_t, but then we need to ensure the generators can deal with this - people volunteering for individual language generators here?