[electrum] move ElectrumWindow.set_contact method to ContactList

Summary:
Remove more code from ElectrumWindow, just keep a signal/slot to let the main window update its other widgets.
There is no change in behavior.

Depends on D14533

Test Plan: same test plan as D14533, plus edit the contact name in the Contacts tab, close and reopen the wallet to verify the updated name is shown

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14534