Improve the ready_dxf module

[ldevillez]

Is your feature request related to a problem? Please describe.
There is multiple improvement to the dxf of module:

• The tool break sometimes the lines of the dxf into multiple parts. Ideally it should connect close edges
• Edge color can be used to order which set of edges must be cutted before another. It should be possible to change the color of contour inside the piece
• When preparing a cutting sheet, we must import each part into a single file. It could be done automatically