qt: Add GUIUtil::bringToFront

Summary:

The sequence show -> raise -> activateWindow is factored out to the new
function GUIUtil::bringToFront where a macOS fix is added in order to
fix #13829.

Partial backport of core PR14123 (commit 5796671)
https://github.com/bitcoin/bitcoin/pull/14123/commits/5796671e1dd8a2d0b1e750c2dce19a10af624095

Depends on D4481.

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/D4482