2.x calendar changes

[MarkBaker]

This is:

- [ ] a bugfix
- [X] a new feature
- [ ] refactoring
- [ ] additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Calendar setting (1900/1904) from a loaded spreadsheet is now saved against the spreadsheet, allowing handling of multiple spreadsheets with different settings, rather than maintained as a global setting. The static global setting is still used for calcuation/formatting, but is set from the spreadsheet when required, then reset afterwards.

This does mean it's dangerous to rely on the old setting, because it is no longer guaranteed to be consistent

[oleibman]

@MarkBaker - this change is probably a fix for issue #1635