[qcad_svg_to_modern_dxf] add option to export separate layers

[vmario89]

The extension processes the file in a whole. So different layers (e.g. with different cutting colors) get merged together. This is a problem in some cases. We could split up the actucal SVG into its layers and then export them to QCAD for each. We could also merge back those multiple files into a common one.

its a lot of work to do, so actually we do it manually.

advantage: merge polylines, remove duplicate lines in one step.