[electrum] move-only: extract MyTreeWidget and related code to its own module

Summary:
util.py has grown big, and the tree widget base class is used for the main widgets in the application (tx history, coins, addresses...) so it deserves to its own module.

The payment request icons are only used in MyTreeWidget or its child classes, so move this as well.
While doing so, make sure the PR_UNPAID, etc... constants are directly imported from paymentrequest.py and not transitively from some gui modules.

Test Plan: Run the application and check for no import error.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: bytesofman, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14167