Unify filters for a model field. Each table view has a list of filters,
one for each associated model. Every filter by default receives a list
of keywords and filters the respective model using a predefined set of
fields.

The user can override the default fields and define its own using the
operator "=".

For example, in a User filter, the query "John" will search by default
in the following fields: first_name, last_name, uuid. On the other hand,
the query "first_name=John" will search only in the field specified at
the left of the "=" operator.

Fix apyrgio/synnefo#179