Support for RADDS Extension Board V3 (and 6 extruders)

- Move FWRETRACT to the planner
- Combine leveling, skew, etc. in a single modifier method
- Have kinematic and non-kinematic moves call one planner method

See https://github.com/MarlinFirmware/Marlin/issues/11103#issuecomment-421387611



Co-Authored-By: tcm0116 <tcm0116@users.noreply.github.com>

Values taken from Zonestar firmware for P802QA with a Melzi board. Timestamps from the firmware package are from April 2016.

`MAX_SERVOS` => `NUM_SERVO_PLUGS`

When possible, make `active_extruder` a `constexpr` to save some PROGMEM.