contact: Implement request-message property

This will be used in handling incoming friend requests.
tox: Add friend request callback

So far it just prints a message.
tox: Add contact-request signal

This will be used for signaling incoming tox friend requests.
contact: Improve tox-id property

NeulandContact: Change the property tox-id to be a pointer to the binary
public key of contact, and introduce a new read-only property tox-id-hex
for the hex string representation of tox-id.