MANUAL_CONTROL message next generation

[hamishwillee]

The MANUAL_CONTROL message was enhanced with support for 16 new buttons and support for 6DOF joysticks in #1674

As part of the discussion it was recognised that the architecture is sub-optimal. In particular the message includes button presses that must be interpreted on the flight stack. This requires the flight stack to maintain a mapping of what the buttons mean. Further it means that the data is included in the message even if it is not needed.

The suggestion was that while button presses might be useful, we could instead have the GCS directly map button presses, and send equivalent MAV_CMDs.

In addition, we might better name and define the DOFs.

The trigger for doing this work would be someone running into limitations from the current message. This is to keep the discussion/need visible.

[stale]

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