NMPObjects are never modified after being put into the cache.
Hence, it is safe and encouraged to just keep a reference to them,
instead of cloning them.

Interestingly, NMPlatform's change signals have a platform_object
pointer, which is not the pointer to the NMPObjects itself, but
down-cast to the NMPlatformObject instance. It does so, because commonly
callers want to have a pointer to the NMPlatformObject instance, instead
of the outer NMPObjects. However, NMP_OBJECT_UP_CAST() is guaranteed
to work one would expect.