move led toggling in main and SlowTicker to IdleLeds module

move fixed led toggling from main loop and SlowTicker::on_idle to a module.
This allows using these leds for different purposes by disabling this module.
Both leds are configurable separately (index starting by 1).
They can be disabled individually by assigning 0.

Default config:

    idleleds.enable    true
    idleleds.led_main  2
    idleleds.led_idle  3