XD-900 Phase I

__DO NOT MERGE UNTIL https://github.com/spring-projects/spring-integration/pull/899 IS MERGED/DEPLOYED__

This is phase I (of II); the SI PR will break the XD build once merged/built/deployed.

This PR must be merged soon after the SI PR (which will be sometime tomorrow).


PropertyAccessors are now inheritable.

PropertyAccessors provided to the IntegrationEvaluationContextFactoryBean
are now named, to enable normal bean inheritance/override semantics.

This commit changes spel-context.xml to configure a Map instead of a List
of accessors.

Phase II will elevate the factory bean to the parent (module common)
context.