Add possibility to digitize a curve from an SVG file without axis

[DunklesArchipel]

Currently the svgdigitizer can only digitize curves, which is defined by two axis.

In principle it would be great a curve from an SVG file could be digitized, which does not have a reference axis. In that case, the scale corresponds to the reference system used to define the positions of objects in the SVG file.

Usage: One could sketch any kind of curve in Inkscape, to illustrate for example a scientific phenomenon. By extracting the x and y data, it would be possible to plot the curve along with other data or simply in a nice way.