[IMP] web: form: labels of several occurrences of a field

Before this commit, when the same field occurred multiple times in
a form view, all occurrences were associated with the same id, and
all occurrences of the corresponding label (if any) had the same
'for' attribute. As a consequence, besides the fact that there were
warnings in the chrome console, clicking on any occurrence of the
duplicated label focuses the first occurrence of the field.

This commit generates a unique id for each pair label/field, when
possible. However, there is a case that can't be handled
automatically: when the label isn't automatically generated (field
located outside a group, or with nolabel attribute set to "1").
Typically, in this case, there is a <label> node with "for"
attribute referencing the associated field. In this situation, the
id must be defined in the arch, and used in the "for" attribute:

  <form>
    <label for="phone"/><field name="phone"/>
    <label for="phone_2"/><field name="phone" id="phone_2"/>
  </form>

Task 2261697