Implementation of the "External Context" abstraction for the RSF environment.
Implementation notes: General search through the SWF codebase has determined
that with a few exceptions, the majority of ExternalContext methods are used
only within the execution.support package, which appears to be for aiding
integration work.
An "environmental" ViewParams adjuster, the purpose of which is primarily
to wrap any URLs being rendered to the client back in a set of SWFViewParams
so that they may access any current flow environment.
A mirror of the builtin SWF "FlowExecutorImpl" construct, modified marginally
to supply a "work payload" represented by a Runnable to be executed before
the flow is closed.
Processing of incoming Spring Web Flow request URLs is represented for a
render cycle as an RSF "ViewParamsInterceptor" which is actually in charge of
invoking* the flow.