Store notifications should accumulate operations and not override them

[boxdot]

Originally posted by @raphaelrobert in #282 (comment)

Currently, when notifying a change about a specific entity, if there is already another change, it will be overridden by the new change. E.g. if entity X is notified about being added, and then being updated, StoreOperation::Add will be replaced by StoreOperation::Update.

In the previous example, this is problem because observers will never see that an entity was added.

A simple solution for that is to introduce an EnumSet and store all operations that were applied to an entity.