overrides: fix callback setter overrides (bis)

overrides: fix callback setter overrides (bis)

The previous commit broke those by trying to pass weak refs
through pygobject, but we should probably have tested the elements
beyond instantiation: weakref.WeakMethod returns a callable, but
that callable when called only returns the ephemeral bound method,
which is the object we want to call, but pygobject has no support
for that.

Instead, fix the memory leaks we were going after by decoupling the
lifecycle of the callback and that of the pad, by passing functors
to pygobject.