Allow user to undo formatting

[impcymru-adam-watkins]

This is technically possible: https://learn.microsoft.com/en-us/office/vba/api/excel.application.onundo

But somewhat nightmare-ish to implement, unless we:

• manually accumulate every single change to the chart and its reverse (which involves writing a wrapper for every type of operation that touches the chart)
• save a copy of the older chart (but where?)

[impcymru-adam-watkins]

Closing as duplicate of #12