Sets an opaque, named pointer on a
GParamSpec. The name is
specified through a
GQuark (retrieved e.g. via
g_quark_from_static_string()), and the pointer can be gotten back
pspec with g_param_spec_get_qdata(). Setting a
previously set user data pointer, overrides (frees) the old pointer
NULL as pointer essentially removes the data stored.
GQuark, naming the user data pointer.
An opaque user data pointer.
The argument can be
The data is owned by the caller of the function.