Add GUI/properties support for customising EMF load/save options

[agarciadom]

We had a question today from a user on how to customise the EMF load and save options for their EmfModel. Our website says this cannot be done, but we do have methods for it in EmfModel since version 2.3.0.

The issue is that this functionality is not exposed from the developer tools, and it cannot be used via typical string properties as we do for other options. This would need to be exposed in some way, but we'd have to find some way to encode the map of options (which may have non-string values) into the StringProperties we normally use.

Once this is corrected, we should also update the Epsilon website, specifically these two articles:

https://eclipse.dev/epsilon/doc/articles/epsilon-emf/
https://eclipse.dev/epsilon/doc/articles/in-memory-emf-model/