Overhaul DeeFilter and DeeFilterModel to work with GI and be more like DeeModelReader and DeeIndex
This branch gives a slight API break in how DeeFilters are created and reverses the order of the arguments to dee_filter_model_new() to match the order in dee_{hash,tree}_index_new(). I tested manually that Python works with DeeFilterModel now.
(resubmitted because of quantum entanglement with the no-const branch). Fixes: https://bugs.launchpad.net/bugs/904293. Appoved by Michal Hruby.