ZF-4704: Refactor Action_HelperBroker to use PluginLoader

Description

Currently, Zend_Controller_Action_HelperBroker maintains its own plugin loading capabilities; this should be refactored to use the PluginLoader for consistency, as well as to benefit from the new include file cache.

Comments

Done in r12111

Changing issues in preparation for the 1.7.0 release.