Handle button interrupts in epicardium + Add button callbacks to pycardium

Depends on/includes !73 (merged)

Parameter passing from core0 to core1 is very ad-hoc and could/should be improved in the future (related to #35)

Shortcomings in micropython:

• Scheduled callbacks are not processed while sleeping (utime.sleep)
• Only one callback is processed per python statement (?)

while True:
  utime.sleep(1) # Only one queued callback event will be handled per loop iteration