[electrum] use the progress bar when broadcasting multiple transactions

Summary:
This can help the user be confident that there is progress when broadcasting multiple large consolidation transactions.

Depends on D14853

Test Plan: Prepare and broadcast a few transactions.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14854