support custom query parameters in QueryBuilder

Add a ``custom_query`` parameter to the ``QueryBuilder._makequery`` method, which allows for run time customization of the stored query, e.g. by request parameters.

Associated branches, which should be merged with this one are:
https://github.com/plone/plone.app.collection/tree/thet-custom_query ... for the plone.app.collection 1.0.x branch
https://github.com/plone/plone.app.collection/tree/thet-custom_query-master ... for the plone.app.collection master branch
https://github.com/plone/archetypes.querywidget/tree/thet-custom_query ... also for plone.app.collection 1.0.x