SortedList possible optimizations

- data structure for storing current sorted state can be transfered into a tree. This would lower the complexity of a single change from O(n*log(n)) to O(log(n)), but raise random access from O(1) to O(log(n)).
- several common operations could be optimized, e.g. clear can removes everything in one notification
- set some threshold when a complete resort would be faster than one-by-one insert sort
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