[CLA][enhancement] automatically use DropDownButton's label as popup's label

Sample representing problem is here:
http://jsfiddle.net/7FVV6/3/

aria-label attribute was incorrectly set as id attribute because of missing label or name
which is not beign provided when using it in scenario such as:

```
<div data-dojo-type="dijit/form/DropDownButton">
  <span>Register</span>
  <div data-dojo-type="dijit/TooltipDialog">Tooltip text</div>
</div>
```

Fixed by adding fallback value representing parents's label.