Split Extrusion width into two options

[mvasilakis]

It would be nice to have more control over extrusion width. One option for extrusion width and a second option controlling the spacing between lines.

For example if we select a .45mm extrusion width we can select 0 for default spacing or enter an offset value like .01/-.01mm to increase or decrease spacing between the lines. Sometimes default is just a tiny bit off one way or another and changing the extrusion width up or down doesn't quite solve the problem.