gui: Favor macOS show / hide action in dock menu

Summary:
Note that macOS toggles between Hide and Show.

Backport of core PR14133:
https://github.com/bitcoin/bitcoin/pull/14133/files

Test Plan:
Cross build for OSX.
Run bitcoin-qt and check the dock icon context menu doesn't display
the show/hide item anymore (see the screenshots from
https://github.com/bitcoin/bitcoin/pull/14133)
Tested on OSX Catalina.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4473