Please consider speeding up the DataObject.files () for the most common cases of
usage by providing lazy Set that will compute its values on demand. So
checkFiles (a method to recognize secondary entries) will not be called until
really needed.

The most common operation is just call files () and do nothing ;-) As is done in
DataNode.getDisplayName () if the underlaying filesystem is local file system (a
FileSystem.annotateName method is called which does nothing). To speed up this
case simple lazy Set is enough.

The second most common operation is files ().iterator ().next () done in
core.projects.SystemFileSystem.annotateName (...) which is not used as offen
(templates and other stuff on default filesystem), but speedup would be nice. To
speed up this case an enhancement to the lazy set that will return
DataObject.getPrimaryFile () as first item of the set is needed and only when
somebody asks for more items the checkFiles is called and rest of the set
initialized.

Please consider making this a new (package private) class in loaders package -
creating new innerclasses in MultiDataObject would probably not be so nice.