Remove the dispose() implementation and replace
with destroy().
This should be promoted as the standard approach
for implementing a composite actor, as it emits a
signal when instances of the actor subclass are destroyed.
Add some extra description to the allocate() function,
explaining how the allocation has to be adjusted to
coordinates relative to the actor as a whole, before
applying to the single child actor it is composed from.
As most actor subclasses will probably want to implement
size requisition, give a simple example of how to do this
on the basis of the composed actor's size, plus some padding.
