Improvements on search/stemming via some heuristics

Not bulletproof, but now heading a direction where (a) exact-prefix
searching is possible (because the FTS is not tokenized via the
stemmer), (b) stemmed variant searching is possible (because the
"fake" TokenizerTable is used to call it up), (c) some heuristics are
performed on the stemmed variant to see if its suitable for searching,
and (d) the search results are culled to strip out any results which
rely solely on "greedy" prefix matching of the stemmed variants only.

Still some cleanup to do.  Also considering making this configurable
by the client (although may not expose this via the UI) so there's
some way to choose between strict/aggressive/forgiving searches w/o
changing consts and recompiling.  Highlighting is somewhat broken at
the moment.