Made some efficiency improvements to NSURL:

1. No need to call Cocoa.StringConstant, so we can simply use plain String constants instead.
2. With that, the KeyList is not necessary any more, either.
3. One of the predefined Keys was wrong (probably copy/paste error).
4. The ResourceValue() function doesn't invoke Cocoa.NSObjectFromNSPtr() any more but returns a Ptr, because the caller knows the expected type anyway, so the caller shall perform this operation.