MAVLink/Cockpit/modifier icons for joystick functions

[ES-Alexander]

• I've used % symbols for modifier here because I think of it as the mod(ulo) operator from Python, but we could choose something else if we can think of something clearer / more intuitive
  • one alternative is using typical symbols based on the modifier name, although I quite like the sense of "groupiness" provided by having a consistent symbol for the different modifiers, just with different numbers