Introduce template and skin manager public API

A new rewritten template manager has been used to fix XWIKI-11193 among other things. It was used since a long time for DW (with a lot of refactoring along the way).

It comes with a lot of new feature that still are internal.
I need to discuss all of them on the mailing list and make them public when I find some time.

New features:

• can use any syntax in the templates (default is old style template behavior)
• can indicate a user to be used to check filesystem template rights
• can indicate that a filesystem template has programming right
• a new rendering macro to include a template

  {{template name="someTemplaye.vm"/}}