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.
oleibman