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/