helper - get only direct children of directory
@param {String} parentId directory id
@param {Object} items files
@returns {Object}  children
function getDirChildren(parentId, items){
    var children = {};
    for(var id in items){
        if(items[id].ancestors[ items[id].ancestors.length-1 ] === parentId)
            children[id] = items[id];
    }
    return children;
}