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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The SVG icon was changed in d3ee556467 but the small XPM icon
was not modified.
Unlike other XPM icons which are used in `ViewProviderSketch.cpp`,
this `PointOnPoint` XPM icon doesn't seem to be used at all.
Nevertheless, we still update it so that the XPM matches
its corresponding SVG.
Also update the `README.md` to explain how these XPM icons
are generated.
