[electrum] Remove shortcut reference to Contacts in main_window

Summary:
Same rationale as D18329: the contacts managers is owned by the wallet, most places that access it are aware of the wallet so we can remove decoupling with the main window.
The PayToEdit widget does not have a reference to the wallet, so just give it a reference to the contact_manager directly (this will remove almost all accesses to self.win when we move the accessed method to the send tab, and the widget will just access its parent API instead of its grandparent's API)

Test Plan:
Open the Contacts tab, add a contact, right click on it and try all the menu action.
Go to the Send tab, type the contact name and check autocompletion works (the contact is suggested, and we can send it funds)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18334