slider: label elements not properly associated with slider control

[Becky Gibson]

If a <label> element is provided for a slider:

<label for="slider1">slider example 1</label>
<div id="slider1" dojoType="dijit.form.HorizontalSlider" .....>
</div>

The label is not properly associated with the slider and thus not spoken by the screen reader. We'll need to use the same process as in combobox to find any label associated with the slider widget, assign a unique id to the label element then use the labelledby property to associate the label element to the component with the role of slider.

[Becky Gibson]

(In [14240]) fixes #7002 Use ARIA to properly associate any label element provided for sliders with the slider focusNode so screen reader will speak the label.

[Becky Gibson]

(In [14241]) refs #7002 - added some labels to test 7002 and forgot to check in