Allow RECEIVER wallet to also produce a payment proof

[dbadol]

Current situation
As of today, only the sender of a transaction can produce a payment proof. However, both sender and receiver have the same information about the transaction and there could be cases where the receiver wants to produce the payment proof.

Solution we propose
Allow the RECEIVER of a transaction to produce a payment proof for that transaction.

Remarks

• As @valdok explained the above is technically possible. But the receiver would need to have stored everything it needs about the transaction (in particular the signature).
• Once implemented, this feature will imply adding the payment proof option to GUI wallets.
• The "Payment proof" as used above would be better decribed as "Transaction proof". We might thus also want to update its name in the GUI...