Adds a weak reference callback to a mini object.
Weak references are used for notification when a mini object is finalized. They are called "weak references" because they allow you to safely hold a pointer to the mini object without calling @ref (gst_mini_object_ref() adds a strong reference, that is, forces the object to stay alive).
MiniObject to reference weakly
callback to invoke before the mini object is freed
extra data to pass to notify