smarter filter operators

[guruofgentoo]

From Ben: "The (slight) complexity of adding additional operators to an existing filter class may point us in the direction of having smarter operators. In this example, a date filter operator could know that it requires no value, know how to generate its own date range, and know how to generate its own query filter."

Correct IMO, this would be a good change to the structure of filter operators. It necessarily makes the operators more specific, so there would likely be more of them.

For what it's worth, the way search works is a kind of a smart operator and could be factored that way. I.e. we could have field-specific search operators that the grid's search uses, and factor away from the current way of setting that up.