Background: There are a number of places in Faces where it needs to distinguish
whether its running in a action/postback request or not. The current (RI)
implementation relies on checking whether VIEW_STATE_PARAM exists in the request
or not. However this doesn't seem to be formalized in the spec.
Need: Formalize this in the spec and/or define something like an IS_POSTBACK
request attribute which can be/is set to flag that this is a postback – and
hence differing impls can use whatever request info they need to make this