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)
adf355849f