Blender Git Loki

Store array of all layer items in the layer tree

Coexists with LayerTree.items and LayerTreeItem.childs listbases which were used for recursive iterations over the layer tree (using BKE_layertree_iterate or BKE_layeritem_iterate_childs). However, it was a bit annoying having to use this everywhere, especially if many args had to be passed as void *. Also, iterative methods with arrays instead of listbases should give us a bit of speedup.
It's possible LayerTree.items will not be needed anymore, but will leave in for now.

Array is only used on BKE level so far.