Fix compilation issue in PropertiesBackingEngine

git-svn-id: https://svn.apache.org/repos/asf/karaf/branches/karaf-2.3.x@1548999 13f79535-47bb-0310-9956-ffa450edef68
Set javax.swing as optional import in the manual

git-svn-id: https://svn.apache.org/repos/asf/karaf/branches/karaf-2.3.x@1549000 13f79535-47bb-0310-9956-ffa450edef68