4.x: Remove handlebars templates from code generators

[tomas-langer]

We should use our class model for generating sources.
It would be useful to change code that is good for templating to string templates in Java, to keep the readability of a template.
If the string templates require additional support in class model, we should add it.

For 4.0, we need to at least deprecate the SPIs that expect usage of templates.