ContentListener should support multiple content keys

[benglass]

Right now the content listener only supports checking 1 content key in the request. The default is for it to check for the cmf main content key where the CMF stores its main PHPCR content document.

If you want to have the content listener automatically load metadata for other objects in the request, you would have register multiple copies of the listener with different content keys for every argument name in any of your actions that represents an SEO object unless you want to always call your action argument $seoContent which is not a desirable restriction.

For example, what if you have a BlogPost object that implements SeoAware? You would have to register a separate content listener which looks at for a $blogPost key in your action. This would not scale very well.

If it supported checking multiple keys then we could allow the user to configure it to listen on 'cmfMainContent' as well as 'blogPost' and 'job' etc. This seems like it might be a nice enhancement actually to allow configuring multiple contentKeys for the content listener.

Original discussion from: #146