Investigate: Change ObserverList to ActionList in Propagation (was ACTS-432)

Original author Andreas Salzburger @asalzbur

Currently the propagation can only have Observers which act on a const cache object, however, changing this to ActionList on a non-const cache would allow, e.g. the MaterialEffectsUpdator to be just an actor in the propagation.

Edited Mar 27, 2019 by Moritz Kiehn

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