LibGUI: Allow FileSystemModel to be rooted one level above "/"

You can now construct a FileSystemModel with a null String() as the
root path. This will root it one level above "/" which makes the
root directory itself selectable as a child.

This will be useful in some places, e.g the FileManager application.

Libraries/LibGUI/FileSystemModel.h

@@ -109,6 +109,7 @@ public:
         RefPtr<Core::Notifier> m_notifier;
 
         int m_error { 0 };
+        bool m_parent_of_root { false };
 
         ModelIndex index(int column) const;
         void traverse_if_needed();