[3dprint] new CURA engine

I just had a look at the new CURA engine code:
https://github.com/Ultimaker/CuraEngine

Nice, elegant C++ implementation. Internally it relies on Clipper, a fast library to perform operations on 2D polygons. That is an excellent choice: Intersecting each 3D triangle with the slice plane produces a bunch of segments. The segments are then linked back together to form polylines and 2D polygons. The Clipper library is used to compute polygon offsets, creating the perimeter and shells.

Now I have to work hard to make IceSL as fast :) There are quite a few places where large speed ups are possible, so we'll see.