[electrum] add a copy_to_clipboard util function

Summary:
This has three benefits:

• factor some boilerplate code
• reduce the dependency on self.main_window in the address tab
• remove a QtWidgets.app reference that does not work with python6

Test Plan:
Go the the address tab, right click on an address and try the 2 copy actions. Test this also with the env var QT_API=pyqt6
Open a transaction dialog (right click on a tx in the history and click Details), select some text in the input widget, right click and "Copy Selected Text"

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18344