Fix of https://github.com/prusa3d/Slic3r/issues/285

Refactored Model.cpp/hpp to C++x11 loops,
simplified the mesh / bbox handling.

utils/view-mesh.pl

@@ -28,9 +28,6 @@ my %opt = ();
 
 {
     my $model = Slic3r::Model->read_from_file($ARGV[0]);
-    
-    # make sure all objects have at least one defined instance
-    $model->add_default_instances;
     $_->center_around_origin for @{$model->objects};  # and align to Z = 0
     
     my $app = Slic3r::ViewMesh->new;