Joystick: support custom MAVLink commands as actions

[ES-Alexander]

QGroundControl supports custom MAVLink joystick actions that are configurable via a JSON file. That seems like it would be a valuable feature to support, and (to my understanding) is likely not too hard to implement (although if we want a properly nice interface for modifying that file, e.g. with auto-generated form fields based on the known message fields, then it could require some extra effort).

This could supersede #825, and is a useful interim while #569 hasn't been completed (although while MAVLink is a key part of our supported vehicles it seems valuable to keep even if #569 gets completed too).