Sorting Facets on CategoryPath (Label) [LUCENE-5370]

Facet support sorting through `FacetRequest.SortOrder`. This is used in the `ResultSortUtils`. For my application it would be very nice if the facets can also be sorted on their label.

I think this could be accomplished by altering `FacetRequest` with an extra enum `SortType`, and two extra `Heap` in `ResultSortUtils` which instead of comparing the double value, compare the CategoryPath.

What do you think of this idea? Or could the same behaviour be accomplished in a different way already?

(btw: I tried building this patch on the trunk of lucene5.0; but I couldn't get the maven build to build correctly. I will try again lateron on the 4.6 branch.

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Migrated from [LUCENE-5370](https://issues.apache.org/jira/browse/LUCENE-5370) by Rob Audenaerde, updated Mar 24 2014

Start by reading FacetRequest.SortOrder and ResultSortUtils to understand the current numeric facet sorting and how CategoryPath values are available. Decide how label sorting should fit the existing API, then verify that facet results can be ordered by CategoryPath label as requested without breaking numeric sorting.

Written by the indexing model from the issue text.