WIP UndoRedo: Added Undo/Redo stack, added Platter::take_snapshot(),

experimental snapshots on loading STLs and increasing / decreasing
model instances.

src/libslic3r/Slicing.hpp

@@ -171,4 +171,9 @@ extern int generate_layer_height_texture(
 
 }; // namespace Slic3r
 
+namespace cereal
+{
+	template<class Archive> void serialize(Archive& archive, Slic3r::t_layer_height_range &lhr) { archive(lhr.first, lhr.second); }
+}
+
 #endif /* slic3r_Slicing_hpp_ */