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.
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