Ports from aria-toolkit and enhancements

Added audit for multiple explicit owners.

Enhanced (and renamed) "non-existent-labelledby" audit to cover all id reference types: 
 * aria-activedescendant
 * aria-controls
 * aria-describedby
 * aria-flowto
 * aria-labelledby
 * aria-owns

Added a few utility methods to support some of the new functionality.
Unit tests for all changes.