[New Feature] Add PWM RC override under joystick buttons

[zdanek]

I'd like to propose a new great feature for joysticks: RC Channel override with set PWM values for buttons.
I've already coded it. So one can choose a new action for particular button and it's a PWM override for channels 8 to 16. Using channels below 8 seems no sense as they are more like throttle, yaw etc.
Since buttons are two state - up and down, a PWM values for up and down are to be set.
Latch mode means that low and high values are toggled (send in cycle) while button press. So first press sends high value, second press send low value and it repeats in cycle. Very useful in some situations.

Here how it looks like

I've created a PR #10431 with this feature.