change the content model of exemplum

[ebeshero]

This is a continuation from #1933 : Now that we have experimented with outputting all exempla from tagdocs (agnostic of language) at https://bauman.zapto.org/~syd/temp/4TEICouncil/html_GLs_all_eg_en/ , we want to try to group examples that are duplicates of each other but described in different languages. We're opening this ticket to work on changing the content model of <exemplum> to permit this.

[jamescummings]

Great idea. 2 output thoughts:

1. Should there be a language identifier supplied in order to indicate what language the example is in (for all languages)
2. Should there be a way to toggle or express interest in seeing examples in all languages vs the current language? (And if sophisticated this could be sticky via a cookie so a user selects once and then all tagdocs pages display all examples unless they change their mind.)

[ebeshero]

Closing because this ticket is outdated because Council decided to introduce an <exemplumGrp> element on ticket #1933, rather than try to allow multiple <egXML> in a single <exemplum>.