we do not have to rebuild indexes after deleting some items
added: DirContainer::DelById(long id)
changed: Dirs::CheckRootDir() addes the root dir if there is no one
added: Dirs::DeleteDir(long id) - deletes specified directory (and its contents)
(from the cache and the database)
added: Db::DelDirById(long id) - deletes a dir from the database
and its first children
added: standard function rm can delete directories
git-svn-id: svn://ttmath.org/publicrep/cmslu/trunk@487 e52654a7-88a9-db11-a3e9-0013d4bc506e