SmbItem.isRootPath returns true for the first subdirectory

[swaechter]

The method SmbItem.isRootPath returns true for the first subdirectory as directory path. A quick fix would be to check if the path is empty, but this won't work on DFS share.

In addition the method (and maybe other methods like getRootPath) should move to the SmbDirectory class. It's not possible to check if a file is the root path (Maybe it's interesting to know if the file is IN the root directory)