Some fixes and cleanup to the Cocoa file drop code
- replaced deprecated NSURLPboardType with newer name, and support it on older SDKs with a #define. Fixes dashboard compiler warnings.
- override the other NSView designated initializer, initWithCoder:, to also register for drop events. Without this, if the view was created in a xib (like in the SimpleCocoaVTK example), drops could not be received.
- accept only drops of file URLs and not other URLs (ex http).
- don't exclude drags that start in the same application
- return YES from performDragOperation: if the drop was successful.
- added more temp variables, for easier introspection in debugger
- added some additional comments.