Layers: Per-Collection edit mode settings

I didn't manage to get the proper object context in the collection
properties editor. That said I got it working for now in a temporary way
since this will change once we get workspaces anyways

(see changes in buttons_context.c and
rna_scene.c::rna_LayerCollection_mode_settings_get)

I still need to handle the merging of the settings. I will find a
provisory solution while we wait for depsgraph.

(also layer_collection_create_mode_settings_object and layer_collection_create_mode_settings_edit could probably be elsewhere - under draw/engines likely)

source/blender/blenloader/intern/readfile.c

@@ -5955,6 +5955,8 @@ static void direct_link_layer_collections(FileData *fd, ListBase *lb)
 
 		direct_link_engine_settings(fd, &lc->engine_settings);
 
+		direct_link_engine_settings(fd, &lc->mode_settings);
+
 		direct_link_layer_collections(fd, &lc->layer_collections);
 	}
 }