Add utilility method to BaseHstComponent to retrieve the default ClientComponentManager

If a developer wants to access the ClientComponentManager, he should be able to retrieve this through a utility method in BaseHstComponent instead of having to write:

ComponentManager compManager = (ComponentManager) this.getServletConfig().getServletContext().getAttribute("org.hippoecm.hst.container.HstContainerServlet.clientComponentManager");

If he did change to have his ClientComponentManager set as a different attr on the servletContext, then he should be able to retrieve the component manager by himself