Define WidgetInterface

* VuFind/Controller/Widget/WidgetInterface.php: New interface.

A Widget is a special type of controller plugin comparable to a web
portal Portlet. It is a self-contained application component that is
interacted and interacts with the main application. The key feature of
a Widget is its capability to represent its internal state as a set of
query parameters.
Define FacetInterface

* VuFind/Controller/Widget/Solr/FacetInterface.php: New interface.
New class: Represent SOLR facet information

* VuFindSearch/Backend/Solr/Response/Json/Facets.php: New
class. Represent SOLR facet information.
Fix path to ZF2 library

* ../tests/unit-tests/phpunit.xml: Fix path to ZF2 library.
Fix docstring of fields property

* VuFindSearch/Backend/Solr/Response/Json/Facets.php: Fix docstring of
  fields property.
Import Zend\Stdlib\Parameters

* VuFind/Controller/Widget/WidgetInterface.php: Import
  Zend\Stdlib\Parameters.