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.

tests/fff_print/test_trianglemesh.cpp

@@ -1,6 +1,7 @@
 #include <catch2/catch.hpp>
 
 #include "libslic3r/TriangleMesh.hpp"
+#include "libslic3r/TriangleMeshSlicer.hpp"
 #include "libslic3r/Point.hpp"
 #include "libslic3r/Config.hpp"
 #include "libslic3r/Model.hpp"