Draft: clean up ViewProviderLinearDimension class
Draft: clean up ViewProviderLinearDimension class

This class handles linear dimensions of edges, or radial
dimensions of arcs and circles.

Various improvements in style, PEP8, spacing, docstrings.
In particular use more readable variable names.

The modules `Part` and `DraftGeomUtils` have been delayed
in import.

The `updataData` method is modified to return immediately
if the Proxy property modified is different from `Start`,
`End`, `Dimline` or `Direction`.

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