Add a third facet item state for expanded hierarchical facets

Looking into this a little further, it looks like this is not really feasible in a widget plugin as the hierarchical structure (and #active) elements are already defined and set in FacetapiFacetProcessor's processHierarchy() method.

I haven't used Apache Solr Search Integration myself so I don't know how compatible this will be. In our case we're using Search API which doesn't actually enforce indexing of parents with child terms. Instead it is available as an optional "Data Alteration" which is performed at index time.

To help aid my description, lets assume the following simple hierarchy:

              parent
        /        |        \
  child1      child2      child3

I should note that we are using OR facets here. I don't think this applies for AND facets.

Search API (with Facetapi's expanded hierarchies option), allows me to set child1 as 'active' without also setting parent as 'active'. More importantly, it includes a filter in the query for child1 without including a filter for parent.

This does not matter whether you enforce that parent is indexed along with child1 or not.

The issue we face is that Facetapi builds the facet hierarchy with both the parent and the child marked as 'active'.

This wasn't a problem before the expanded hierarchies as you couldn't facet on a child without activating the parent first. Also treating the parent as active is necessary from a ordering perspective; child1's hierarchy needs to float to the top of the list with other active terms. But we need a way of distinguishing (inactive) parent from (active) child1 for display purposes.

So I am proposing a third state which would be a 'highlighted' state. This would be for parent items who have active children, but are not really active themselves.

From an implementation stand point, this state doesn't actually need to be explicit in code, we just need the ability to detect it from within a widget plugin. Here is a rough example patch based on beta7 that my colleague created this afternoon.

Whew! It sure is hard to describe these things clearly. I hope that all somewhat made sense, let me know if you see any glaring holes in the logic.