Add ModelArrange.hpp as extension to Model.hpp, use it for duplicating

Refactored Arrange interface: remove the union based BedShapeHint, replace it with proper function overloads

WARN: this commit is only intermediate, it does not compile.

src/libslic3r/CMakeLists.txt

@@ -120,6 +120,8 @@ add_library(libslic3r STATIC
     Line.hpp
     Model.cpp
     Model.hpp
+    ModelArrange.hpp
+    ModelArrange.cpp
     CustomGCode.cpp
     CustomGCode.hpp
     Arrange.hpp