[FIX] web: editable list: shift-tab and readonly fields

Before this rev., navigating with shift-tab didn't produce the
expected result when there were readonly fields on the way. For
instance, pressing shift-tab on a field:
- to the right of a readonly field: the focus was kept in the
  current field, whereas it should have been moved to the first
  editable field to the left of the readonly one
- in the editable first cell of a row, with a readonly field in
  the last column: the focus was moved to the previous line,
  which is ok, but on the first editable cell of it, whereas it
  should have been on the last editable cell.

@KangOl this commit needs to be forwardported up to saas-12.2, not
later (the code slightly changed in saas-12.3, and we did another
version of the fix).