When loading an AMF file having multiple objects that look like multiple parts of a single object, prompt user and ask how to consider it. #2970

Conflicts:

	lib/Slic3r/Model.pm

lib/Slic3r/Format/AMF/Parser.pm

@@ -143,6 +143,7 @@ sub end_document {
         }
         
         foreach my $instance (@{ $self->{_instances}{$object_id} }) {
+            next if !defined($instance->{deltax}) || !defined($instance->{deltay});
             $self->{_model}->objects->[$new_object_id]->add_instance(
                 rotation => $instance->{rz} || 0,
                 offset   => Slic3r::Pointf->new($instance->{deltax} || 0, $instance->{deltay} || 0),