Styling DesThree expressions through DOM editing is not effective

[jared-hughes]

A few issues here that are related:

1. (reported by @wompking) expressions that go out the top/bottom of the screen are removed in the DOM and do not get the custom DesThree styling (cube on the left "@3" characters removed. For example, try scrolling down on https://www.desmos.com/calculator/kwroojtkoq (especially on a shorter screen)
2. Ctrl+A on an expression adds "@3" to the end (rather, DesThree doesn't remove the "@3" because there is no graphChanged event); purely visual. For example, try pressing ctrl+A in expression 2 of https://www.desmos.com/calculator/ghjdxovcyj
3. Collapsed folders (#21). This might not be related because it affects functionality and not visuals.

Two possible fixes to consider:

• use CSS selectors like [expr-id=24] > ... to control style. This would be convenient, but it has the downside of not updating autoOperatorNames
• use a MutationObserver on the expressions list to observe when expressions are changed, then check if the data property expr-id falls within the set of DesThree expressions

I'm leaning toward the second because it might fix #9 too.

[jared-hughes]

Issue also applies when duplicating an expression through the Desmos button. All expressions lose DesThree styling, and the "@3" appears; this persists until graph state changes.