[backport#14594] qt: Fix minimized window bug on Linux

Summary:
a88640e123ca0c00d81719f9a009699c26e85b90 Fix minimized window bug on Linux (Hennadii Stepanov)

Pull request description:

Fix #14591

On some Linux systems the minimized to the taskbar (iconified) main window cannot be restored properly using actions from the systray icon menu when `QSystemTrayIcon::contextMenu()` is a child of the main window.

Backport of Core PR14594

Test Plan:

ninja all check check-functional
./src/qt/bitcoin-qt -regtest

make sure minimizing and restoring the window works fine

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8546