Joystick: Add feature to assign a function i.e. modes to buttons

Add feature to be able to assign functions like a mode to a button

The issue does not identify any files, tests, or entry points. First locate the joystick button handling and mode-assignment code in the Qt ground-control station, then clarify which functions and modes must be assignable and how assignments are configured; done means the requested functions can be assigned to buttons and work correctly.

Written by the indexing model from the issue text.