Allow producing "payment proofs" for offline transactions too

[dbadol]

Current situation
As of today (v7.5) Payment proofs can only be produced for online transactions.

Solution we would like
From the user's point of view, the need of having a payment proof is independent of the type of transaction (online / offline) that was used. It would thus make natural sense to be able to produce such proofs for both online and offline transactions.

Remarks
Ask explained by @valdok, such a proof for an offline transaction is possible. But it's a different kind of a proof than the ones for online transactions. Nonetheless, he confirms that the possibility of such a proof was indeed designed into the protocol.

[Maxnflaxl]

#1826