Dxf writer full refactor

[DomCR]

Description

Complete refactor for the Dxf writer to make sure that the entities are written in a sorted way.

Tasks done in this PR

• Tables section
• Blocks section
• Entities section
• Objects section

Related Issues

#163 fix the polyline Dxf format in the writer

TODO

• Seqend management #168 Fix Seqend management
• Finish complex entities:
  • Dxf writer Hatch #172
  • Dxf/Dwg writer MText #171
  • Mesh (needs full implementation)
  • Dxf writer MLine #170
  • Solid3D (needs full implementation)
• Add writer for ExtendedData
• Objects not implemented:
  • AcdbPlaceHolder (needs full implementation)
  • Material (needs full implementation)
  • SortEntitiesTable
  • Scale
  • VisualStyle (needs full implementation)