BUG: DirectoryListView - Allow path with same root to be added

Before this commit, if two existing paths with the same root were added, the
second one wasn't considered.

For example, after successively calling "addDirectory" with the
following paths:

 "/tmp/foo"
 "/tmp"

, the second path "/tmp" wasn't added.

This commit fixes the "hasDirectory()" method so that it uses an "Exact"
match instead of "StartWith" match.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22356 3bd1e089-480b-0410-8dfb-8563597acbee