Add Template Editor within FACE Editor support

[steve-hickman-epistimis]

FACE Templates are completely embedded in strings in FACE UoPMessageTypes. They have their own syntax. To ensure they are well formed, we really want the Template editor to be used when editing this string.

It may be possible to learn how to do this by looking at how Sirius and XText are integrated:
See

• https://altran-mde.github.io/xtext-sirius-integration.io/userguide/
• https://github.com/altran-mde/xtext-sirius-integration/
• https://blogs.itemis.com/en/domain-specific-visualization-of-textual-models-with-xtext-and-sirius