qt: Use GUIUtil::bringToFront where possible

Summary:
Partial backport of core PR14123 (commit 6fc21ac):
https://github.com/bitcoin/bitcoin/pull/14123/commits/6fc21aca6d5e16c3ece104fec8e5b3df116893b4

Depends on D4482.

Test Plan:
Build and run bitcoin-qt on OSX and Linux.
Check the dock icon is working as expected.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4483