Implement a first version of the class editor that will be used by App Within Minutes

This class editor will be used to define the structured data handled by the XWiki applications created with App Within Minutes. The editor will automatically generate and update the class sheet and class template.

The design can be found at http://incubator.myxwiki.org/xwiki/bin/view/Improvements/ApplicationWithinMinutes . For implementation details, see http://lists.xwiki.org/pipermail/devs/2011-October/024440.html .