Refactoring. Use Model class for representing the plate in GUI

xs/xsp/TriangleMesh.xsp

@@ -24,6 +24,8 @@
     void rotate(double angle, Point* center);
     TriangleMeshPtrs split();
     void merge(TriangleMesh* mesh);
+    ExPolygons horizontal_projection()
+        %code{% THIS->horizontal_projection(RETVAL); %};
 %{
 
 SV*
@@ -159,6 +161,16 @@ TriangleMesh::bb3()
     OUTPUT:
         RETVAL
 
+Polygon*
+TriangleMesh::convex_hull()
+    PREINIT:
+        const char* CLASS = "Slic3r::Polygon";
+    CODE:
+        RETVAL = new Polygon ();
+        THIS->convex_hull(RETVAL);
+    OUTPUT:
+        RETVAL
+
 %}
 };