Fire PropertyChanged event outside of Computed update method

This was implemented for CollectionSlot before, but I added similar logic to AtomSlot, because that one can also create accidental dependencies when bound to ContentControl. ContentTemplate in ContentControl gets instantiated during PropertyChanged event, which caused accidental dependencies for AtomSlot. While doing this, I've also moved some shared code from CollectionSlot and AtomSlot up to MemberSlot.