Layers: use IDProperty and override collection properties system

First this replace a custom data struct with IDProperty, and use
IDProperty group merge and copying functions. Which means that a collection
property setting is only created if necessary.

This implements the "Layer Collection settings" override system, as
suggested in the "Override Manifesto" document.

The core is working, with Scene, LayerCollection and Object using a
single IDProperty to store all the render settings data. Next step is to
migrate this to depsgraph.

Note: Clay engine "ssao_samples" was hardcoded to 32 for now. It will come
back as part of "Workspace Settings" later.

Many thanks for Bastien Montagne for the help with the UI template
nightmare ;)

Differential Revision: https://developer.blender.org/D2563

source/blender/editors/include/UI_interface.h

@@ -948,6 +948,7 @@ void uiTemplateHeader3D(uiLayout *layout, struct bContext *C);
 void uiTemplateEditModeSelection(uiLayout *layout, struct bContext *C);
 void uiTemplateReportsBanner(uiLayout *layout, struct bContext *C);
 void uiTemplateKeymapItemProperties(uiLayout *layout, struct PointerRNA *ptr);
+void uiTemplateOverrideProperty(uiLayout *layout, struct PointerRNA *collection_props_ptr, struct PointerRNA *scene_props_ptr, const char *name, const char *custom_template);
 void uiTemplateComponentMenu(uiLayout *layout, struct PointerRNA *ptr, const char *propname, const char *name);
 void uiTemplateNodeSocket(uiLayout *layout, struct bContext *C, float *color);
 void uiTemplateCacheFile(uiLayout *layout, struct bContext *C, struct PointerRNA *ptr, const char *propname);