Use Pax CDI capabilities to detect web bean bundles

Pax Web currently uses the Pax-ManagedBeans manifest header as an indication to wait for enrichment of the servlet context during initialization.

This manifest header has been replaced in Pax CDI 0.4.0 by an OSGi extender requirement, see
https://ops4j1.jira.com/wiki/display/PAXCDI/Requirements+and+Capabilities.

From now on, Pax Web should check for the same requirement instead of checking for Pax-ManagedBeans.

As before, this is a manifest-only operation. There are no compile-time dependencies on Pax CDI.