qt: Replace QRegExp with QRegularExpression

Summary:

qt: Add a function that extracts the suffix from a filter

Extract the 'Extract first suffix from filter pattern...'
functionality into a testable utility function

qt: Replace QRegExp with QRegularExpression

Co-authored-by: Pavol Rusnak <pavol@rusnak.io>
Co-authored-by: Jarol Rodriguez <jarolrod@tutanota.com>

test, qt: Add tests for GUIUtil::extractFirstSuffixFromFilter

This is a backport of core-gui#620
Depends on D18177

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18178