Rework the setting of custom date / time format preferences:
- fix bug causing case changes if custom format case insensitively
matches a drop down entry;
- fix invalid format examples in tooltip;
- update URL for the format documentation;
- add support for quoted literals to the format validity warning.
From discussion in
https://github.com/subsurface/subsurface/issues/3849#issuecomment-1481239270.
Signed-off-by: Michael Keller <github@ike.ch>
