[animation] Allow registering custom progress function

[animation] Allow registering custom progress function

A ClutterInterval can change the way the progress is computed
by subclassing and overriding the ::compute_value() virtual function.

It should also be possible to register a custom progress function
in the same way it is possible to register a custom transformation
function between two GValues.

This commit adds an internal, global hash table that maintains a
GType <-> progress function association; each ClutterInterval
will check if there is a progress function registered for the
GType of the initial and final values of the interval and, if
it has been found, it will call it to compute the value of the
interval depending on the progress factor.