Colors as assigned to imported drawing entities if they are not merged
with other entities.
The code has been refactored to remove much duplication in reading of
attributes.
The code gives brief messages on the Python console about unsupported
DXF festures and also issues some new errors.
There is no support yet for making colors 1-9 and 250-255 contrast with
the creeen background color. Colors are generated by code rather than a
lookup table; this code can eventually modify the colors it generates to
contrast with a specific background color.
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