Create enumerated class for the access levels

In the current version access levels ("edit", "admin", "programming", etc) are strings retrieved with XWikiRightServiceImpl.getRight(String action) and all comparisons are made using "equals" on strings. It is somehow unsafe since an extra space, typo or wrong case would produce errors. Also, an enumerated model would allow comparisons to be made using "==" which improves the readability of the code and the performance.