Replace hardcoded 'XWiki' prefix with preferences

The XWiki space is used as a hardcoded standard for all "under the hood" purposes. All the users and groups are in there, the global rights, administrative, register and login pages, skins, classes , templates and sheets, etc. Now, it is pretty hard to move some of these elements somewhere else, if not impossible (without making major modifications in the java code).

My intention is to create and use several variables that specify the default space for:

• Users (a list of places to search for users will be even better, but complications arise)
• Groups
• Administration pages
• Classes (if a classname is required without specifying the space, first search here), templates and sheets
• Skins
  If such a preference is not specified in xwiki.cfg, use 'XWiki' as the default (ensures compatibility).

This feature would be hard to implement, as this implies changes in many places, both java and velocity (for all skins).