Nepomuk FolderSelectionModel: Improve checked state

Mark certain folders as PartiallyChecked. We now use the following
criteria -

* Qt::Checked - When the directory and all its subdirectories are
                included
* Qt::PartiallyChecked - When a directory is included but its
                sub-directories contain an excluded directory
* Qt::Unchecked - When a directory is excluded

BUG: 291397
FIXED-IN: 4.10