Joystick: allow the user to specify the "stopped" value for each axis

We already support specifying min/max - it could also be useful to specify the stopped/neutral value for smarter messages when pilot input is not being received but the user has elected to continue sending messages (i.e. a slightly more advanced version of #713).

We could try to infer some of this from firmware versions, but allowing specifying the values manually is valuable for custom vehicles and joystick protocols, especially for future support.

Start by locating the joystick handling and the existing min/max axis configuration, then review issue #713 for the related behavior. Done means users can specify a stopped or neutral value for each axis and receive smarter messages when pilot input is absent but message sending continues, including for custom vehicles and joystick protocols.

Written by the indexing model from the issue text.