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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I recently upgraded to `shiboken2-5.14.2.1`, and this broke preprocessing of
`src/Gui/WidgetFactory.cpp` because `SHIBOKEN_MICRO_VERSION` is now defined as
`2.1` (WTF?!), making it a floating-point literal which the preprocessor does
not like. It is only used for version comparison with `5.12.0`, so replacing
`SHIBOKEN_MICRO_VERSION` with `0` should be OK.