Add a "PSBT Operations" dialog, reached from the "Load PSBT..." menu
item, giving options to sign or broadcast the loaded PSBT as
appropriate, as well as copying the result to the clipboard or saving
it to a file.
This is a backport of core#18027 [3/4]
https://github.com/bitcoin/bitcoin/pull/18027/commits/a6cb0b0c29d327d01aebb98b0504f317eb19c3dc
Depends on D11981