Add joystick button limit

[dheideman]

Limits the maximum number of joystick buttons for which functions can be added on vehicle settings, joystick setup page, to 16.

Prevents issues of unconfigurable buttons outside the range 0~15 from appearing to be configurable, such as: https://cdn-business2.discourse.org/uploads/bluerobotics/original/2X/8/886612ffc44d2d46447e44976e857bec158ec7f3.png

Closes bluerobotics#21

[hamishwillee]

@dheideman @DonLakeFlyer Is this just sub, and in what version will this appear? Probably a useful note to add to the joystick docs

[dheideman]

This was written for sub but should also apply to any other systems using joysticks in a similar fashion.

[jaxxzer]

@hamishwillee The constraint is the 16 bit field in the MANUAL_CONTROL message where the button values are sent (to all types of vehicles). So this should affect all vehicles.

[hamishwillee]

Thanks guys. So would it be reasonable to add the following comment to the docs (for the current version/now)

"Up to 16 joystick buttons can be assigned to vehicle functions (If more than 16 button checkboxes are displayed only the first 16 will be used)"

[jaxxzer]

@hamishwillee That would be correct for 3.2. After this merge, only 16 max will ever appear for assignment.

[hamishwillee]

Thanks @jaxxzer . I ended up adding the following note "Select the flight modes/vehicle functions activated by each joystick button. A maximum of 16 joystick button actions can be set.".

This works for all versions, because even if more are displayed at least it is clear why button 17+ does not work.