Improve diffs in SceneInspector

[johnhaddon]

It would be great to have the character level differences highlighted rather than just the whole thing.

It seems that difflib's SequenceMatcher().get_opcodes() method might be handy here - the actual HtmlDiff class seems a bit rubbish though.

[andrewkaufman]

It would be nice if the collapsible sections indicated that a diff exists within. So any attribute diff would result in some marking on the >Attributes label.

[johnhaddon]

That sounds good - I was already planning on putting a summary on the section labels in the same way we have them for the Attributes and Options nodes but hadn't considered a diff there as well. Maybe just the number of common attributes in white, and the number of differing ones each way in red and green?

[andrewkaufman]

Yeah, that sounds good to me.

[johnhaddon]

I've split the section header bit out into #911, as it's orthogonal to this one, which was about formatting of the diff text.