[electrum] refactor text completion

Summary:
Move the text completion code from the paytoedit widget into its own class and module, so it can be reused for other widgets such as the seed editor.

This is a partial backport of electrum#4033

Test Plan:

python -m electrumabc_gui.qt.completion_text_edit

Run the application and check that auto-completion still works in the Pay-To field (add a few contacts to the wallet)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17568