Allow Sparse infill density field to be expressed as % or pitch in mm

[dromanovich]

In some 3D printing applications, the infill percentage is not important to the design of the part. For example, some infill patterns can be used for cosmetic or functional purposes. I’d like to be able to specify the pitch of the infill pattern (if and whenever possible). Knowing the pitch and line width allows designer to precisely control the size of the infill pattern. Think nozzles, fan grills, filter covers, sieves, decorative elements etc...

For example, instead of 13%, the sparse infill density filed may say 7mm.

As a stretch goal, I think it would be neat to specify X and Y pitch independently. It may look like this:
7 X 3mm.

I am a huge fan of the multipurpose UI input fields. This keeps UI clean yet powerful.