Download Signed PSBT From Signing Flow

Currently, once you have a signature, the only way to "transfer" it is to copy the signature array. A PSBT is a much more portable way to share this signature. A second button that says "Download Signed PSBT" and adds the signature to the unsigned one for download would be pretty straightforward with existing psbt tooling. I think this should be relatively straightforward to put together for someone looking to get familiar with the code or just knock out a quick win.