Some work on refactoring the ReferenceHelper to extract the logic for updating cell references.

[MarkBaker]

This is:

- [ ] a bugfix
- [ ] a new feature
- [X] refactoring

Checklist:

• Changes are covered by unit tests
• 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?

Even absolute cell references ned updating in Conditional Formatting rules when new rows/columns are inserted; and the ReferenceHelper currently only updates relative references.

This is a preliminary step toward allowing updates to absolute cell references, required to update Conditional Formatting rules when columns/rows are inserted/deleted as highlighted by Issue #2678; the first step in a multi-step process.

• Step 1
  Refactor the cell reference update logic to make it easier to modify, and to test the planned modifications
  This may also make the code more performant, as it allows elimination of a lot of code duplication in the different methods to update different components of the worksheet.

• Step 2
  Modify the cell reference update logic to accept a flag indicating whether it should update absolute as well as relative cell references

• Step 3
  Add the logic to update Conditional Formatting ranges and rules when inserting/deleting rows/columns