Fix incorrect class type for widgets.
Fix inability to get View instance from widget registry.

Even though View instance was added to registry it could not be retrieved as
WidgetRegistry::_resolveWidget() throw exception if object did not implement
WidgetInterface.
Fix expected exception message in test.
Add "Widget" suffix to widget classes.
Do instanceof check when adding to registry instead of when accessing.
Move default widgets list from WidgetRegistry to FormHelper config.
Merge pull request #5040 from cakephp/3.0-widget-registry

3.0 widget registry