Add naming strategy for @MVC request mappings.

This change adds a strategy for assigning a default name to an
@RequestMapping controller method. The @RequestMapping annotation
itself now has a name attribute allowing the explicit assignment
of a mapping name.

This is mainly intended for use in EL expressions in views. The
RequestContext class now provides a getMvcUrl method that internally
delegates to MvcUriComponents to look up the handler method.

See the Javadoc of MvcUriComponents.fromMappingName.

Issue: SPR-5779