In previous optimization in outliner I assumed that order in treehash was not important.
But testing outliner in datablocks mode revealed a problem: when user expands multiple recursive levels and then closes any element, it always closed the top level of recursion.
Now it should work fine with recursive trees.
Now treehash contains groups of elements indexed by (id,nr,type). Adding an element with the same (id,nr,type) results in appending it to existing group. No duplicates are possible in treehash.
This commit should also make lookups a little bit faster, because searching in small arrays by "used" is faster than searching in hashtable with duplicates by "id,nr,type,used".
