Support setting the root spring app context as parent of the HST spring context

I think it's a good idea to be able to set the root Spring application context as the parent of the default context that HST creates and uses. 

A use case for this is for example when using Spring security and you want to access say a UserManager bean in your HST components, which is a bean defined in the root context. It would be nice to have access to that and I think it's trivial:

• Add a conf property 'component.inherit.root.web.application.context' = true (false by default)
• Modify org.hippoecm.hst.site.container.SpringComponentManager#initialize to use the 3 argument constructor of DefaultComponentManagerApplicationContext (org.hippoecm.hst.site.container.DefaultComponentManagerApplicationContext(ComponentManager componentManager, ContainerConfiguration containerConfiguration, ApplicationContext parentApplicationContext)), allowing the WebApplicationContext be the parent of HST ComponentManager, if the option is set to true.

wdyt?