Refactoring. Use Model class for representing the plate in GUI

xs/xsp/ExPolygonCollection.xsp

@@ -68,5 +68,15 @@ ExPolygonCollection::append(...)
             THIS->expolygons.push_back(expolygon);
         }
 
+Polygon*
+ExPolygonCollection::convex_hull()
+    PREINIT:
+        const char* CLASS = "Slic3r::Polygon";
+    CODE:
+        RETVAL = new Polygon ();
+        THIS->convex_hull(RETVAL);
+    OUTPUT:
+        RETVAL
+
 %}
 };