LibGUI: Start working on a GFileSystemModel and hook that up in FileManager.

This is a read-only model for the tree view, at least initially. We'll see
where we take it from there once it's more polished.

LibGUI/GTreeView.cpp

@@ -87,7 +87,7 @@ GVariant TestModel::data(const GModelIndex& index, Role role) const
 }
 
 struct GTreeView::MetadataForIndex {
-    bool open { true };
+    bool open { false };
 };