Unstitch and Reverse Normal (#856)

Unstitch takes a body's faces out as surface bodies; Reverse Normal turns a surface body's faces over. Both are saved in format 18. Closes #766.

crates/doc/src/clipboard.rs

@@ -321,6 +321,8 @@ impl FeatureKind {
             | K::SheetConvert { body, .. }
             | K::SurfaceExtend { body, .. }
             | K::SurfaceTrim { body, .. }
+            | K::Unstitch { body, .. }
+            | K::ReverseNormal { body, .. }
             | K::MeshConvert { body, .. } => mapped_name(body, bodies),
             _ => {}
         }