Introduce a configuration to select the HTML version to use according to the current skin

With XRENDERING-137, we now have an HTML5 renderer.

Problem: the XHTML 1.0 renderer is used in a lot of places in XWiki Platform, and hardcoded.

We have 2 choices:

1. introduce a new constant: XWIKI_SYNTAX, that we could use everywhere
   or
2. introduce a new configuration (that could depends on the skin maybe, since Colibri is still using XHTML 1.0) to change globally the default syntax to use.