Allows the GUI to clear settings.json file and save settings.json.bak file when
GUI "Reset Options" button is pressed or -resetguisettings command line option
is used. (GUI code already backs up and resets the "guisettings.ini" file this
way, so this just makes the same behavior possible for "settings.json")
This is a backport of core#15936 [3/3]
https://github.com/bitcoin/bitcoin/pull/15936/commits/f9fdcec7e932843a91ddf7f377e00bd2a6efb82a
Depends on D12605