add Leds module for central management of leds, add post codes to main

completely redesigned leds module as discussed in #424
- replaces led code (and gpio) in main.cpp and SlowTicker.cpp
- adds POST codes to main.cpp
- POST leds can be configured separately
  via space separated pin descriptions
- uses Pin to have maximum flexibility
- manages the four standard leds and play led
- dimmed mode shows that events are running without distraction
  (otherwise led is bright or off, kind of blinks while activities)
- also respects leds_disable setting
- free form pin configuration
    e.g. leds.pin_idle  4.28! blink
    or   leds.pin_idle  blink 4.28!

config like current led behaviour:

    leds.enable        true                       # can be omitted
    leds.pin_main      blink                      # main loop
    leds.pin_idle      blink                      # idle event
    leds.pin_gcode     off                        # no gcode led
    leds.pin_play      off                        # no play led

example config:

    leds.enable        true                       #
    leds.pin_main      dimmed                     #
    leds.pin_idle      blink                      #
    leds.pin_gcode     4.28!                      # use play led for gcodes
    leds.pin_play      off                        #
    leds.pin_sdok                                 # default
    leds.pins_post     1.21 1.20 1.19 1.18 4.28!  # POST leds reversed + play led for highest bit
    play_led_disable   true                       # PlayLed is replaced by Leds