Support export pattern to DXF.

[Roman Telezhynskyi]

AutoCAD DXF (Drawing Interchange Format, or Drawing Exchange Format) is a CAD data file format developed by Autodesk for enabling data interoperability between AutoCAD and other programs.

Useful links:

• AutoCAD DXF and dxflib
• dxflib library on github
• What is dxflib?
• dxflib Downloads
• dxflib Documentation
• Qt Paint System
• QPaintEngine Class
• QPaintDevice Class

By using Qt Paint System we can easy to add support for DXF files. Main issue is understand how dxf work.