Make it possible to configure the "expanded" icon on folderish document types

If the user has selected the "folderish" facet (or if it is inherited) it should be possible to configure the expanded icon.
You can add the following help label: "This icon is displayed in the left tree if the node of the type being configured here is "open", so that children are displayed. This is an optional refinement: you can use the same icon in any case."