Until now, the program used to open one popup per transaction when broadcasting multiple transactions, which can be annoying when broadcasting many transactions.
Fix this by summarizing the result of the broadcast in a single messagebox.
ref T3352
This has also the benefit of no longer using the main_window.broadcast_transaction method, which could have side effects if the user was in the middle of dealing with a payment request before deciding to send broadcast a bunch of unrelated transactions.