[electrum] fix integer overflow for the Proof total amount

Summary:
PyQt translates python integers to signed 32 bits int when emitting a int in a signal.
We need to specifically emit a 64 bit integer. See https://doc.qt.io/qt-5/qtglobal.html#quint64-typedef

Also apply the same fix to the two signals that I previously fixed in a different way (with a larger performance overhead) in https://github.com/Bitcoin-ABC/ElectrumABC/pull/291

Test Plan:
load a proof with a total stake of more than 21,474,836.47 XEC, check the amount is displayed correctly.
Select coins and addresses of various amounts, check the selected amount displayed in the status bar.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14781