Support reading of binary DXF files

What does the merge request do?

Closes #400

Also fixed a small detail that leading spaces should not be deleted inside text entity specifications for ASCII files.

Implementation notes

I've tried to change as little as possible in the existing code. However this extension would love some refactoring. Possibly we could use ezdxf for reading and writing of DXF and add the SVG conversion on top of it.

Test files were thankfully provided by @Mannshoch.

Summary for release notes

Binary DXF files (from Autodesk R13 onwards) can now be imported into Inkscape.

Checklist

• Add unit tests (if applicable)
• Changes to inkex/ are well documented
• Clean merge request history