Edit Widget field binding popup: unavailable fields are not displayed as unavailable

Steps to reproduce:

1. On a layout, drop the Contributors widget on a cell.
   -> The layout widget editor popup is displayed.
2. Click on the Edit button of the Field.
3. Move your mouse over simple fields such as Title, Description, or Created.
   -> No sign is displayed that these fields won't work.
4. Try to click on them.
   -> Nothing happens.

A side bug is that if you click on an authorized field such as Subjects, both Subjects and Contributors are displayed as selected.