Feature/Improvement: A buyer scans Sell-Offer.psbt (1x vin / 1 vout) coin inputs and change vout needs to be added

[NiKrause]

A potential buyer of a NameOp or NFC scans an already partially(!) signed Sell-Offer (PSBT) with one NameOp-Input and one Output for the Seller with the required price!

When this is scanned by a DOIWallet

• check signature exists and is valid from NameOp-VIN
• add coin input from own wallet which is sufficiently paying the requested amount in the first output
• add change output
• display PSBT QR-Code
• broadcast to Doichain Network
• Test Segwit and Legacy