Modified NSWindow, added example

- Added properties to NSWindow for BackingScaleFactor, DocumentEdited,
MovableByWindowBackgound,
  RepresentedFile, RepresentedFilename, and RepresentedURL.
- Added Constructor and Operator_Convert to NSWindow to allow
assignment of a native window.
- Added NSObject.RespondsToSelector method.
- Added NSWindowExample window.
- Modified convenience methods to take advantage of new NSWindow
properties.