Use GClosure variants for Alpha
Use GClosure variants for Alpha

The Alpha class allows setting custom functions instead of the
logical id for the easing mode. The upstream C API has GClosure-based
variants that allow GObject-based language bindings to avoid a lot of
the gory details of creating closure-like data structures.

PyClutter should just provide clutter.Alpha.set_func() and the global
clutter.alpha_register_func() as wrappers around the GClosure-based
C API.

pmarti/pyclutter,pmarti/pyclutter,GNOME/pyclutter,pmarti/pyclutter,GNOME/pyclutter,pmarti/pyclutter