Initial commit for category collation framework

Initial commit for category collation framework

Hidden behind $wgExperimentalCategorySort until it's reasonably
complete.  If that's false, no behavior should change (but I didn't test
carefully, so poke me if there's a bug).  See DefaultSettings.php for
documentation on setting it to true.  Currently you should not do this
except if you're working on the feature, since functionality is not
close to reasonable yet and will change rapidly.

Bug 1211 is already fixed with this commit for me.  However, many other
things still need to be done, so this is all very much a
proof-of-concept.