Blender Git Loki

Refactor how LayerTile is bound to LayerTree/LayerTreeItem

Main benefit of this commit is that layer group relations and layer order are stored in LayerTree now (blendkernel struct), not in structs of the layer manager editor.

Two things I'm still not happy with:
* LayerTreeItem (blenkernel struct) now stores the LayerTile (layer manager editor struct) as void pointer. We need this to access the LayerTile from the LayerTreeItem, without expensive lookups. This could be solved by storing a hash table of LayerTiles though.
* Added BKE_layertree_iterate which is a handy util, but we need it pretty often so we also need to add lots of structs for custom data storage. For order/group independent iterations we could use the hashtable mentioned above and iterate over it.