[2.0.1] Implement some methods for compatibility with keypad

Implement methods in the queue for keypad compatibility: get_into and overflowed
Use the original key number in the hash of an event to make it compatible with 1.0.

Previous update:

An event's key_number is no longer equal to the underlying event's key number, but is offset by the number of keys of the previous keypad in the list, so that the key numbers now are 0 to N like a normal keypad.
The original value can be accessed with original_event.key_number

The MultiKeypad class now gains a key_count property.