Incrementally update NestedArrays by GraphChanges

[fdietze]

• Only calculate the needed updates lazily (maybe use Observable?)
• Actually use it somewhere
• Integrate Filters
• Things that don't need a traversal can be modeled with immutable hashmaps: (Notify, Member, Bookmark, Author, Read, ...)
• Measure how much slower a traversal of Array[Array[Int]] is compared to NestedArrayInt. If it is not much slower, use Array[Array[Int]], as it is easier/faster to update.