New switch action: ACTION_FIXED_THROTTLE_VALUE

Sets a fixed throttle value via a long press of a switch.
The throttle value is reset once the switch is released.
Useful for triggering the starting aid, too.

The used power is configurable via the new
"fixed_throttle_in_watts" config.h entry.

Side note: The new action was almost called ACTION_TURBOBOOST_THROTTLE :)
Reset throttle_stat if SUPPORT_THROTTLE is disabled

This is needed to switch off ACTION_FIXED_THROTTLE_VALUE.

Don't do it for KINGMETER and BMS type displays though:
We can't ensure there is serial data to interpret on
every loop run, so we would make the starting aid act flaky.