[electrum] move format_amount* functions from main_window to amount.py

Summary:
This does not need to be a method associated with a GUI window. Moving it to another module will reduce the dependencies on main_window by subwidgets of the main window.

Another small effort in making main_window less of a God object

Depends on D18316

Test Plan:
python test_runner.py unit

Run Electrum ABC, check for no errors and look at some amounts (XEC and fiat), double-click on a transaction and on an address to open the modified dialogs

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18317