dxf input: add polyline support

Jonathan Neuhauserrequested to merge
joneuhauser/extensions:dxf-fix-polyline-nomura into master

In the chat, a user named Nomura added a patch for polyline support. I've merged their file with master.

The code could possibly be refactored to use the ValueConstruct structure better.

Unit test sample file again provided by Nomura.

Edited by Jonathan Neuhauser

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