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

Create a watch-only descriptor wallet in bitcoin-qt, import a pubkey in the console with the importdescriptors RPC, send fund to that address, wait for the funds to become available, go to the send tab, create an unsigned transaction, save it to file.

Use the new menu "File > Load PSBT from file" to inspect the unsigned transaction.