gui: Drop ShutdownWindow dependency to BitcoinGUI

Summary:
Remove circular dependency "qt/bitcoingui -> qt/utilitydialog -> qt/bitcoingui"

This concludes backport of Core PR18036 [2/2]
https://github.com/bitcoin/bitcoin/pull/18036/commits/3aee10b80b9d9a0f5172fc2ee75f03a37d5c3863

Test Plan: ninja && src/qt/bitcoin-qt

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8697