Better Handling of legacyDrawing Xml

[oleibman]

Fix issue #4105. Passing through form controls from load to save was part of the limited support added with PR #3130. However, the legacyDrawing vml, which is a crucial piece of that change, often contains a description only of notes (comments) with no form control contents. Since legacyDrawing is constructed by Xlsx Writer when it doesn't already exist, Xlsx Reader should not interfere with that ability - it should save the contents of legacyDrawing only when it appears to not consist merely of note descriptions.

It should also be an option to delete the legacyDrawing vml before saving. This allows one to add comments, at the cost of losing form controls. A method deleteLegacyDrawing is added to Spreadsheet to permit this.

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.