change handler concept needs overhaul

[rlindner81]

using change handler to write back the new value you get is too tempting. this will lead to errors when a scoped change is performed and then again may lead to errors when deletions are propagated to subscopes.

all in all, the change handler should probably just be called for the root scope to avoid confusion. or maybe we have a scopedChangeHandler for consciously handling scoped changes. this needs conceptual work.