line weight of 0mm is resolved as 0.02mm in the frontend of the drawing add-on
line weight of 0mm is resolved as 0.02mm in the frontend of the drawing add-on

A line weight of 0mm is resolved as 0.02mm, this prevents backends
to draw nothing if line weight is 0mm.
Valid DXF line weights are fixed values: 0mm, 0.05mm, 0.09mm...
fixes #226

mozman/ezdxf,mozman/ezdxf,mozman/ezdxf,mozman/ezdxf,mozman/ezdxf