CAY-1841 Filters for Left-hand project navigator (kind of like Eclipse panel filters) to show/hide everything/objentities/dbentities/queries

patch by Ilya Vasiuk (original by Artiom Kravchenko)

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1515560 13f79535-47bb-0310-9956-ffa450edef68