Allow extending the set of templates, instead of overwriting it

Currently the model explorer only supports a single templates directory, meaning that it either uses all of the built-in templates, or a completely custom set of templates loaded from somewhere else.

It would be great to be able to load additional project / model specific templates, while still using the built-in templates for all the elements that aren't handled by the project specific ones. Currently this is only possible by making a copy of the built-in templates.