WIP: Reworked slicing

1) Slicing code moved to TriangleMeshSlicer.cpp,hpp from TriangleMesh.cpp,hpp
2) Refactored to use as little as possible of admesh.

src/slic3r/GUI/MeshUtils.hpp

@@ -3,6 +3,7 @@
 
 #include "libslic3r/Point.hpp"
 #include "libslic3r/Geometry.hpp"
+#include "libslic3r/TriangleMeshSlicer.hpp"
 #include "libslic3r/SLA/IndexedMesh.hpp"
 #include "admesh/stl.h"
 
@@ -12,9 +13,6 @@
 
 namespace Slic3r {
 
-class TriangleMesh;
-class TriangleMeshSlicer;
-
 namespace GUI {
 
 struct Camera;