Allow adding multiple brim layers

[RetromanIE]

It would be great to add an option to set the brim layers within the "Skirt and brim" area of "Print Settings". In S3D this is/was called "skirt layers" in the "additions" tab of the S3D print settings and was very useful when printing thin rectangular shapes that must be printed upright rather than flat on the bed(lithophanes for example).

When printing something like a lithophane, having the option to add a 3 or 4 layer brim really helps with bed adhesion and improves print quality by reducing wobble. You might think it would be harder to remove, but 3 brim layers peels off prints easier and cleaner than a single standard brim.

Adding this option in would really be a great help to us lithophane printers as well as anyone looking for that extra bit of bed adhesion.

An example of multiple brim layers in S3D.

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[neophyl]

Duplicate of #2605