Attachments: Undo Support

[jleandroperez]

We currently support editing NSTextAttachment subclasses: ImageAttachment + HTMLAttachment.
However, there's no undo support. Let's revisit the API!

[designsimply]

I was unable to confirm the ability to undo attachment modifications using the following testing steps:

1. Open a post for editing.
2. Insert an image.
3. Tap the image, tap again for options, select "Details."
4. Make changes to the web display settings for the image and tap "Done."
5. Shake the device to see undo options.
6. Check to see if the image edits were undone.

Result: the image attribute options I changed (right-alignment and thumbnail image size) were not undone after I completed the undo option.

^{Tested with design5279.wordpress.com using WPiOS 8.2.0.20170731.}

[jleandroperez]

@designsimply thank you for spotting this regression!! i've filed an issue in the WPiOS repository with your repro steps. Fix is on the way, and should make it into WPiOS 8.4.