Allow search filtering on custom index

When defining a filter form (on content views and advanced search), even when using a generic widget, it's not possible to use a custom field that would not be in a document schema.

For instance, when creating a new index for fulltext, it's not possible to search on it (see doc at http://doc.nuxeo.com/x/W4ao#Fulltextqueries-UsingfulltextqueriesinNXQL for custom fields available for full text queries)