💡 Support for multi colored SVGs

[MrShark]

Discussed in #39

Use case

Nesting svg-objects that are multicolored to represent various types of cuts on a laser cutter (Ie. black for outer contours, blue for inner contours, red for engraving, green for etching etc.).

Problems

• Etchings that are closed curves are picked up as holes in the parts (and small parts are placed inside).

• The color information is partly lost when exporting (I think when merging lines)

Solution idea

A new setting Cutting Colors where all colors found in the imported parts are displayed and can be selected as outlines of the parts.
Merging of curves should only be done with the cutting colors.