Initial rewrite of MultiBackend driver with separate functions for all I...

...LS driver methods. Fix to checkCapability to work properly with drivers using __call() magic method and more data to supportsMethod() for better decision-making including the possibility to use supportsMethod also without _call().

Currently MultiBackend driver checks the driver it uses for support for holdings and some recently added functionality (storage retrieval requests, ILL requests) and tries to return something sensible even if they are not available. If you feel this should be extended to all functions, I can do that too.