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.