The "Merge layer duplicates" Tree view context menu option for LayerContainers did not work.
Additionally:
- Using `openTransaction` and `commitTransaction` is required for the `merge_by_name` function but also for the `add_layer` function. Otherwise undo-redo does not work properly.
- Have replaced `translate("Draft", ...)` with `_tr(...)`.
Move `make_layer` to `draftmake`; `Layer` and `LayerContainer`
to `draftobjects`; `ViewProviderLayer` and `ViewProviderLayerContainer`
to `draftviewproviders`.
The make function and the classes are imported in `Draft.py`
to support the usage of the older `VisGroup`.
