Pressing Enter in a document containing a selectManyDirectory removes its items

When creating/editing a document with a selectManyDirectory widget withing the form (e.g. the Subjects property), pressing Enter has an unexpected behavior:

1. Enter a document edition/creation form
2. Add one or more items to the Subjects property
3. Go to any "input type="text"" form part (e.g. the Title property)
4. Press enter
Expected: The form is submitted
Actual result: The first of the subjects is removed.

Accidentally triggering this can eventually result in unnoticed loss of data.

A workaround is to include the attached template (fix_enter.xhtml) as part of the layout, using JS to force the behavior of the inputs.

The actual patch would have to fix how the selectManyDirectory widget works, by making it use an "input type="button"" (I think?) or an "a" link.