CAY-1954 Make Cayenne class constructor protected

also adding @SupressWarnings where those were needed in Cayenne.java
CAY-1943 XML file not deleted when a DataMap is deleted from the project

Pull request by Savva Kolbachev - https://github.com/apache/cayenne/pull/13
Added by aadamchik: project.getUnusedResources().clear();
CAY-1943 XML file not deleted when a DataMap is deleted from the project