Sets an opaque, named pointer on a ParamSpec.
The name is specified through a Quark (retrieved e.g. via
g_quark_from_static_string), and the pointer can be gotten back from the this with
get_qdata. Setting a previously set user data pointer, overrides (frees)
the old pointer set, using null as pointer essentially removes the data stored.
the ParamSpec to set store a user data pointer
a Quark, naming the user data pointer
an opaque user data pointer