Render Layers and Collections (merge from render-layers)

Design Documents
----------------

* https://wiki.blender.org/index.php/Dev:2.8/Source/Layers

* https://wiki.blender.org/index.php/Dev:2.8/Source/DataDesignRevised

User Commit Log
---------------

* New Layer and Collection system to replace render layers and viewport layers.

* A layer is a set of collections of objects (and their drawing options) required for specific tasks.

* A collection is a set of objects, equivalent of the old layers in Blender. A collection can be shared across multiple layers.

* All Scenes have a master collection that all other collections are children of.

* New collection "context" tab (in Properties Editor)

* New temporary viewport "collections" panel to control per-collection
visibility

Missing User Features
---------------------

* Collection "Filter"
  Option to add objects based on their names

* Collection Manager operators
  The existing buttons  are placeholders

* Collection Manager drawing
  The editor main region is empty

* Collection Override

* Per-Collection engine settings
  This will come as a separate commit, as part of the clay-engine branch

Dev Commit Log
--------------

* New DNA file (DNA_layer_types.h) with the new structs
  We are replacing Base by a new extended Base while keeping it backward
  compatible with some legacy settings (i.e., lay, flag_legacy).

  Renamed all Base to BaseLegacy to make it clear the areas of code that
  still need to be converted

  Note: manual changes were required on - deg_builder_nodes.h, rna_object.c, KX_Light.cpp

* Unittesting for main syncronization requirements
  - read, write, add/copy/remove objects, copy scene, collection
  link/unlinking, context)

* New Editor: Collection Manager
  Based on patch by Julian Eisel
  This is extracted from the layer-manager branch. With the following changes:

    - Renamed references of layer manager to collections manager

    - I doesn't include the editors/space_collections/ draw and util files

    - The drawing code itself will be implemented separately by Julian

* Base / Object:
  A little note about them. Original Blender code would try to keep them
  in sync through the code, juggling flags back and forth. This will now
  be handled by Depsgraph, keeping Object and Bases more separated
  throughout the non-rendering code.

  Scene.base is being cleared in doversion, and the old viewport drawing
  code was poorly converted to use the new bases while the new viewport
  code doesn't get merged and replace the old one.

Python API Changes
------------------

```
- scene.layers
+ # no longer exists

- scene.objects
+ scene.scene_layers.active.objects

- scene.objects.active
+ scene.render_layers.active.objects.active

- bpy.context.scene.objects.link()
+ bpy.context.scene_collection.objects.link()

- bpy_extras.object_utils.object_data_add(context, obdata, operator=None, use_active_layer=True, name=None)
+ bpy_extras.object_utils.object_data_add(context, obdata, operator=None, name=None)

- bpy.context.object.select
+ bpy.context.object.select = True
+ bpy.context.object.select = False
+ bpy.context.object.select_get()
+ bpy.context.object.select_set(action='SELECT')
+ bpy.context.object.select_set(action='DESELECT')

-AddObjectHelper.layers
+ # no longer exists
```

source/blender/editors/include/ED_view3d.h

@@ -37,7 +37,7 @@ struct BMEdge;
 struct BMFace;
 struct BMVert;
 struct BPoint;
-struct Base;
+struct BaseLegacy;
 struct BezTriple;
 struct BoundBox;
 struct EditBone;
@@ -187,7 +187,7 @@ void pose_foreachScreenBone(
 void ED_view3d_project_float_v2_m4(const struct ARegion *ar, const float co[3], float r_co[2], float mat[4][4]);
 void ED_view3d_project_float_v3_m4(const struct ARegion *ar, const float co[3], float r_co[3], float mat[4][4]);
 
-eV3DProjStatus ED_view3d_project_base(const struct ARegion *ar, struct Base *base);
+eV3DProjStatus ED_view3d_project_base(const struct ARegion *ar, struct BaseLegacy *base);
 
 /* *** short *** */
 eV3DProjStatus ED_view3d_project_short_ex(const struct ARegion *ar, float perspmat[4][4], const bool is_local,
@@ -353,7 +353,7 @@ struct ImBuf *ED_view3d_draw_offscreen_imbuf_simple(
         int alpha_mode, int samples, bool full_samples, const char *viewname,
         struct GPUFX *fx, struct GPUOffScreen *ofs, char err_out[256]);
 
-struct Base *ED_view3d_give_base_under_cursor(struct bContext *C, const int mval[2]);
+struct BaseLegacy *ED_view3d_give_base_under_cursor(struct bContext *C, const int mval[2]);
 void ED_view3d_quadview_update(struct ScrArea *sa, struct ARegion *ar, bool do_clip);
 void ED_view3d_update_viewmat(struct Scene *scene, struct View3D *v3d, struct ARegion *ar, float viewmat[4][4], float winmat[4][4]);
 bool ED_view3d_quat_from_axis_view(const char view, float quat[4]);