Merge #11338: qt: Backup former GUI settings on -resetguisettings

Summary:
723aa1b qt: Backup former GUI settings on -resetguisettings (Wladimir J. van der Laan)

Pull request description:

Writes the GUI settings to `guisettings.bak` in the data directory before wiping them. This can be used to retroactively troubleshoot issues (e.g. #11262) where `-resetguisettings` solves the problem.
(as discussed in yesterday's IRC meeting)

Tree-SHA512: c64f5052d992eb02057ba285435f143c42d0cc456144a4c565e1c87be833737f9df750d0aee10810f85047c820d9b4f9f22fd94a6f09f4b28a9cf41b63a56586

Backport of Core PR11338
https://github.com/bitcoin/bitcoin/pull/11338/

Test Plan:

make check
./bitcoin-qt -resetguisettings
guisettings.ini.bak should exist in your .bitcoin dir

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3934