Adds ability to Filters to signal to their form field whether Filter has
been used.

This is achieved by adding is_used property to Filter that also sets
Filter.field.is_used. Filters should set this property accordingly in
their filter method.
To ensure that Filter.field IS the same object Form operates with, adds
FilterSetFormMixinthat is used as first base when crating
FilterSet.form. This mixin negates effect of Form deepcopying its fields
at form instantiation.

Addresses: #118