Separate User macros from internal non public system macros

Right now there's only a single macros.vm file containing all macros. I think it would be good to have 2 files:

• one for user macros
• another one for macros that are used internally but that shouldn't be used by users directly. Anyone using those macros would do it at their own risk as they wouldn't be part of an exposed API and we would be allowed to modify them without going through a deprecation cycle. Also these macros would't need to be documented on xwiki.org