WL#828: Combined BTREE/FULLTEXT indexes

It is rather typical to have queries like

"Where group_id=5  and  match(name) against ("search")"

can't be executed very efficiently as MySQL has to use ether index on group_id
or full-text index, but not both, while selectivity of each of them could
be not enough for optimal performance.

We shall add possibility to add combined indexes which Feature BTREE (or possibly
RTREE) functionality allowing to perform full-text search on items selected by
these indexes.

This is very useful for all "catalog" type applications which usually allow limit 
search on current group as well as some other restrictions.