Improve management of widgets bound to a schema field

There are several issues with widgets bound to a schema field:

• the binding is not visible on the widget configuration
• when using a template widget on a complex property, subwidgets palette is not shown.

=> solutions:

• make the binding visible on widget configuration (read only for now, as making it possible to edit it would require modifying the list of available widget types for this field)
• make it possible to use the template widget on any document widget