GTreeView: Support multiple root-level items

Previously it was only possible to have a single root-level item in a
GTreeView. This was an oversight and I didn't realize it because this
code was only ever used in the FileManager, which has one root ("/").

Also factored out item toggling into a separate function, and increase
the base indentation level so that root items can be toggled as well.

Finally, let the user toggle the selected item with the spacebar. :^)

Libraries/LibGUI/GTreeView.h

@@ -27,6 +27,7 @@ private:
     int toggle_size() const { return 9; }
     int text_padding() const { return 2; }
     void update_content_size();
+    void toggle_index(const GModelIndex&);
 
     template<typename Callback>
     void traverse_in_paint_order(Callback) const;