More options for HTTP cache settings

Currently the content generated from .vm templates can either be non-cached (default), or cached (actually, browser's default cache settings), depending on whether there's a headers_nocache long property in XWiki.XWikiPreferences. There should be more options possible:

• always allowed to be cached publicly
• allowed to be cached privately
• never stored, not even in the back-forward cache

Additionally, since adding this property in XWikiPreferences beefs up a class that's already close to the limit of a class definition, the cache should also be configurable in xwiki.cfg, under the xwiki.httpheaders.cache key.