Add a new 'terminal' xproperty in TemplateProviderClass

It would be nice to be able to specify if a template defined by a template provider should be used on a terminal or a non-terminal document.

This is supposed to be just a hint that an advanced user can override from the create page UI if he wishes, but otherwise, it should default on the hint's value.

This is also useful in the transition period towards Nested Documents, since the existing page templates should be marked as terminal to ensure they work as non-updated applications expect them to.