ui: signer should check if the entered psbt has more signatures than the original

[edouardparis]

From feedback: Also, can you check if the number of entries partial_sigs strictly increased and if not tell them that "No new signature was inserted in the PSBT" ?

[darosior]

Note to the reader: it was in the context of a vault acknowledgement / activation. Conceptually we'd like to sanity check that the user did append their signature to the PSBT. Technically we cache the PSBT we sent them so we can check that the one they send back contains a new signature (partial_sigs field).