Date and/or time fields: inputs other than the first don't have a label

[avernet]

This is to be fixed for:

• xf:input bound to a node of type xs:dateTime
• fr:dropdown-date
• fr:fields-date

The label for the control points to the first input, but other inputs don't have a label, which is an issue per WCAG2 1.3.1 (A), reported as E885 by TotalValidator.

A label can't point to multiple form fields. Ideally, we would generate:

<fieldset>
    <legend>Birthday</legend>
    <label for="day">Day</label>
    <input id="day">
    <label for="month">Month</label>
    <input id="month">
    <label for="year">Year</label>
    <input id="year">
</fieldset>

But that might be too much of a chance and create too much trouble for styling. So instead, we could generate:

<span role="group" aria-labeledby="label">
    <span id="label">Birthday</span>
    <input id="day" title="Day">
    <input id="month" title="Month">
    <input id="year" title="Year">
</span>

Also see this article, which has more about this technique, including a link to an example, which works well for me with VoiceOver.

[avernet]

We should first do #2219, so we can have a <span> for the label of the control.

[avernet]

1230ae9 and 47b9c41 take care of adding titles for the fr:dropdown-date and fr:fields-date. For the xf:input, we need to figure where the resources should come from.