Checkboxradio: Moving label-finding code into one function

This helps render the code unit-testable.
Checkboxradio: Rely more on native means of getting the label

Improves performance a great deal when the .labels property is supported, and
also improves performance when such a property is not supported (FF27).

http://jsperf.com/checkboxradio-label-retrieval-non-nested/5

Closes gh-7293
Fixes gh-7292