Make it easier to assign a flow definition an identifier when registering it in a registry

Currently, there is no way to assign an flow definition an explicit identifier when registering it using the convenient Spring 2.0 flow namespace flow:registry tag. When using this tag, flow definition identifiers are engineered entirely from convention by stripping off the .xml file extension. If you do not like this convention, you must configure a XmlFlowRegistryFactoryBean manually. This ticket should investigate better ways to allow an explict flow id to be assigned.