For HippoFacetNavigation beans, add a utility method to get the count of all child HippoFacetNavigation beans combined

I'll add:

/**

• Return all the hippo:count values of all child HippoFacetNavigationBean combined. This count can be different from the getCount() of this
• {@link HippoFacetNavigationBean} because of two reasons:
  * <ol>
  * <li>A document in the resultset can belong to multiple child {@link HippoFacetNavigationBean}

  's</li>

• <li>A document in the resultset can belong to zero child {@link HippoFacetNavigationBean}'s: this happens for example when
  * the document has a property, say 'my:date' , but its date does not belong to any of the ranges that the child {@link HippoFacetNavigationBean}

  represent</li>

• </ol>
• @return Return all the hippo:count values of all child HippoFacetNavigationBean combined.
  */
  Long getChildCountsCombined();