XForms support for multiple labels

[avernet]

Currently, we support external labels, as in:

<xf:label for="my-input">Address</xf:label>
<xf:input ref="name" id="my-input"/>

But we don't support having several such external labels. For instance we don't have a way to tie the labels "Billing" and the label "Address" to the input field in:

<xf:label>Billing</xf:label>
<xf:label>Address</xf:label>
<xf:input ref="name" id="my-input"/>

We could do this either by supporting multiple labels points to a single field, or by supporting a way for fields to point to their labels. The latter technique seems preferable as it allows the form author to specify an order for the labels. Also, it is in line with what is being done in HTML. If we name the attribute labeled-by, the above example with the input field pointing to the two labels would look like:

<xf:label id="billing-label">Billing</xf:label>
<xf:label id="address-label">Address</xf:label>
<xf:input ref="name" labeled-by="billing-label address-label"/>

[ebruchez]

Changing the title to reflect the fact that we might want to support a combination of (probably a single) internal and external labels for #4491.