This sets an opaque, named pointer on a miniobject.
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 until the this is
disposed. Setting a previously set user data pointer, overrides (frees) the old pointer set, using
null as pointer essentially
removes the data stored.
destroy may be specified which is called with
data as argument when the this is
disposed, or the data is being overwritten by a call to set_qdata with the same
A Quark, naming the user data pointer
An opaque user data pointer
Function to invoke with