Handle downgrade/upgrade from p tags

[caffodian]

Use setCustomData to preserve a heading id when converting to a non-heading element, and restore that to the actual id if that element is ever changed back.

[sbaughman]

Did some investigation into this issue and here are some findings:

1. Default behavior is pretty good. Switching headings to paragraphs and back does not trigger the ACF and lose the id, even if other actions are done in between.
2. Auto-save and manually saving also does not trigger the ACF, so an autosave will not lose the id.
3. What does lose the id is toggling the source dialog, but in that case all of the elements are re-rendered anyway, so customData and expando data also do not work.

At this point, it seems that the way to allow data to persist through a toggle of source dialog or other such actions would be through storing heading data in a variable in the plugin itself. However, all of the more obvious cases for id persistence are already covered by default behavior. I think in these more complex cases, it would be little better than a guess as to whether persisting an id was actually the author's intention, at least until we get some user feedback

[caffodian]

Thanks for the writeup! Good summary of stuff we discussed.

Given that the major broken case seems to be source dialog (and I assume setData), we should just punt on this. Someone would have to de-header something and re-header it via the source dialog (or afterwards) for the id to go missing, which seems quite edgy.