[electrum] move new contact dialog to contact_list.py

Summary:
Move some code out of the main_window god object.
There is no change in behavior.

Depends on D14532

Test Plan:
run ./electrum-abc, go to the contacts tab, right click and select "Add Contact - Address"
Fill the form (try both a valid and invalid address)

Redo the same test with the Wallet > Contacts > New menu

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14533