* clutter/clutter-behaviour.h: Add a function prototype
for the foreach function: we need something more specific
than GFunc; add clutter_behaviour_get_actors(), used to
get a list of the actors.
* clutter/clutter-behaviour.c: Add debugging notes; add
a warning for behaviour implementations missing the
alpha_notify vfunc; add api documentation
* clutter/clutter-behaviour-opacity.c:
* clutter/clutter-behaviour-path.c:
* clutter/clutter-behaviour-scale.c: Reimplement the
alpha_notify functions using the new foreach function
and, where possible, by directly iterating on the
actors: this shaves off the number of recalculations
of the property/alpha values, and the number of
functions.