Improve portlet integration

Include in the XWiki Platform some of the more generic changes required to make it work in portlet mode using https://github.com/xwiki-contrib/sandbox/tree/master/xwiki-portlet (which is the second integration approach described in http://dev.xwiki.org/xwiki/bin/view/Drafts/PortletIntegration#HLoosely-Coupled ).

Note that the goal of this issue is not to make XWiki work in portlet mode as is. You still need to make specific changes, some described in https://github.com/xwiki-contrib/sandbox/blob/master/xwiki-portlet/README.txt and some committed in feature-portlet-3.5.x branch (see https://github.com/xwiki/xwiki-platform/compare/stable-3.5.x...feature-portlet-3.5.x ).

Also note that these changes don't mean we decided on the Loosely-Coupled portlet integration approach. They just make it easier to use https://github.com/xwiki-contrib/sandbox/blob/master/xwiki-portlet/ but otherwise they are generic and could benefit both approaches.