slider: add aria-valuetext attribute

[becky]

In FF3 it's now possible to expose the text equivalent for the current value using the optional aria-valuetext attribute. The numeric aria-valuenow attribute is still always required even if aria-valuetext is used.

For example: <div role="slider" aria-valuemin="1" aria-valuemax="3" aria-valuenow="2" aria-valuetext="medium"> If aria-valuenow changes to 1 or 3 then aria-valuetext should be changed as well.

If the value point is just a number anyway, then aria-valuetext is not necessary. and the attribute need not be present. Mozilla will fall back and using aria-valuenow for text of the value if no aria-valuetext attribute is present.

Need to investigate if this can be added or not.

[becky]

sent mail to a few contributors on 2/12/08 - waiting for feedback

adding ARIA valuetext attributes to the slider. This would be used when the slider values are not numeric as in the small, medium, large example on forms/test_Slider.html. We can give the screen reader the actual text value (in addition to the numeric valuenow) so that it can be announced. This would make the text label type value more usable for screen reader users.

The easiest implementation would be to support only when discreteValues is specified as an integer and the label count matches the discreteValues count. I looked through the code and it seems too difficult to discover any labels since if there were two sets of labels I would not know which to use (and supporting RTL complicates that further). One option is to add a parameter to the labels to specify the "default" or "priority" labels. I am open to other suggestions.