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/Polygon.cpp

@@ -48,12 +48,12 @@ int64_t Polygon::area2x() const
 }
 */
 
-double Polygon::area() const
+double Polygon::area(const Points &points)
 {
     size_t n = points.size();
     if (n < 3) 
         return 0.;
-
+    
     double a = 0.;
     for (size_t i = 0, j = n - 1; i < n; ++i) {
         a += ((double)points[j](0) + (double)points[i](0)) * ((double)points[i](1) - (double)points[j](1));
@@ -62,6 +62,11 @@ double Polygon::area() const
     return 0.5 * a;
 }
 
+double Polygon::area() const
+{
+    return Polygon::area(points);
+}
+
 bool Polygon::is_counter_clockwise() const
 {
     return ClipperLib::Orientation(Slic3rMultiPoint_to_ClipperPath(*this));