GeoGebra via material ID

[Rob Beezer]

At

https://github.com/PreTeXtBook/pretext/pull/2238

Brian Walton has used the pre-processor to convert a "simple" GeoGebra
#interactive with a material-id into a more capable #interactive using a #slate
child. That gets us a better rendering (under the hood). Very nice idea.

Code has been merged, but the sample article only really has one test example.

Sample article has some new information on optional control elements.

Subsection 15.1: GeoGebra
https://pretextbook.org/examples/sample-article/html/section-interactive-server.html#subsection-geogebra-server

* Please keep an eye out for problems, or better, do some testing. A change
like this really does not belong on pretext-announce but I don't want
non-developer authors to get caught out either.

* It'd be nice to have a few examples that make use of additional attributes
that affect the rendering. If you have examples like this, consider donating
them to the sample article?

Thanks,
Rob