Improve dirty detection

[aduth]

Updated: While the original issue below has since been resolved, we should investigate cases where changes may be unintentionally reflected in a new post. For #1059, I suspect it was because the list block was setting attributes immediately upon becoming mounted, causing the editor to consider itself dirty. We should look to make dirty detection more durable.

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Original issue:

Steps to reproduce:

1. Visit Gutenberg page

Expected result: There are no changes indicated.

Actual result:

[nylen]

It's a bit disheartening to see this happening already, even though we are taking a very simple approach to dirty detection. Some related discussion on other issues/PRs:

• Add basic 'dirty state' handling #610 (comment)
• Fix a few issues with saving post titles, dirty handling, etc. #848 (comment)

Also, I'm not really sure how to debug this, due to the extremely annoying #919.

[aduth]

The original steps are now resolved as of #1059, but I'm going to keep this open and rename the issue, as I'd like to consider how we might make dirty detection more durable.