Support distance constraint in 3d

[hlorus]

[amrsoll]

Is this about making class_defines.SlvsDistance future-proof with 3D coordinates or implementing a distance constraint with elements outside of a 2D sketch?

[hlorus]

This is about making class_defines.SlvsDistance work with 3d entities (entities outside of a sketch). There's not too much left, it is even already working besides the display of the constraint. So what's left is the SlvsDistance.matrix_basis() to to return a correct matrix if the constraint holds 3d entities.