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.
| this |
the ParamSpec to set store a user data pointer |
| quark |
a Quark, naming the user data pointer |
| data |
an opaque user data pointer |