Objective-C memory management policy specifies that methods named with
alloc, new, copy and mutableCopy are the ones that return objects with
refcount 1.
Additionally, `@property (..., copy, ...)` only modifies the setter; the getter
still returns the object as-is. While it should be correct to create
a new immutable object every time, we should not create an object
with refcount 1, but an autoreleased object.
Implement the NSPasteboardItem class. For now, it remains relatively
useless since NSPasteboard does not yet know how to use it. However, it
should be pluggable into NSPasteboard once NSPasteboard learns about
NSPasteboardReading and NSPasteboardWriting.
