Base list refcount with Bastien Montagne

Fix reading collection objects and small refactors here and there

collections.objects.link()

Use BLI_genericNodeN

Layers doversion: use 1, 2, 3, ... for new collections

RNA: layer.collections[2].objects

This does not include objects from nested collections.
Missing bits: objects.link, objects.unlink

Fix mem leak for recursive collections and get layer.bases and objects
to work

(also some base util functions, and a refactor to unify the functions
for nested and non nested collections)

Cleanup: silence unused prop warning

Finish nested collections rna (recursions ftw)

Fix nested collection write/read

Fix crash when dupli scene

Layer collection nesting (DNA/RNA)

Layer collection initial RNA

new/remove/active

BKE_scene_ layer/collection API touch ups

Fix compiling with Cycles enabled

Brings back some code that was removed in 17beb573563c8a4 to avoid naming conflicts. Now did some renaming to avoid these.

Merge branch 'blender2.8' into layers

Use enum instead of preprocessor defines

Add/use DNA_DEPRECATED_LAYERS

Using DNA_DEPRECATED resulted in thousands of warnings, so rather not use this. These warnings can still be handy though to check for usage of deprecated struct members as we go, so this quick way to toggle them could be handy. Note that this should eventually be replaced by DNA_DEPRECATED once stuff is actually deprecated.

Initial RNA for scene layers

Next will be the collections, and then the other missing props of rna_def_scene_layer

Note: it would be nice to have a way to tag rna properties asa
deprecated. For example, scene.engine can still be valid (and mapped to
scene.layers.active.engine), but it would be nice to indicate the addon
writers of the new API.

Fixup for versioning: name initial layer ... "Layer"