[alpha] Allow registering alpha functions

[alpha] Allow registering alpha functions

In order to unify alpha functions and animation modes in ClutterAlpha
we should be able to register alpha functions and get a logical id
for them; the logical id will then be available to be used by
clutter_alpha_set_mode().

The registration requires API changes in ClutterAlpha constructors
and methods. It also provides the chance to shift ClutterAlpha
towards the use of animations modes only, and to alpha functions
as a convenience API for language bindings alone.