Retitling Cloned Worksheets

[oleibman]

Fix #641 (marked stale in 2018, but now reopened). When a sheet's title is changed, PhpSpreadsheet updates references to the old sheet name found in formulas. Which is a good idea when the sheet is attached to the spreadsheet, but a bad idea when it isn't (often because it has been cloned without re-attaching to the spreadsheet). This PR continues to change formulas in the former case, but will no longer do so for the latter.

This is:

• a bugfix
• 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 and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.