Add DraftObject and ViewProviderDraft to serve as
the parent classes of all Draft objects
and all Draft view providers.
Inside `Draft.py` we need to import
`_DraftObject` from `draftobjects.base`;
and `_ViewProviderDraft`, `_ViewProviderDraftAlt`,
and `_ViewProviderDraftPart` from `draftviewproviders.view_base`.
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