#543 file corruption: polyline width removed

[efa]

opening the file:
https://sourceforge.net/p/librecad/bugs/_discuss/thread/296ab497/6d12/attachment/LibreCAD_DXFlinesTickness.dxf.bz2
and resaving, the file become corrupted as polyline width are removed.
If LC do not support polyline width, it should ignore it leaving untouched the property.
See related bug:
https://sourceforge.net/p/librecad/bugs/520/