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/ui-egui/src/timeline.rs

@@ -12,6 +12,8 @@ pub(crate) fn icon_of(k: &FeatureKind) -> &'static str {
         FeatureKind::Sketch { .. } => "sketch",
         FeatureKind::Extrude { .. } => "extrude",
         FeatureKind::SurfaceExtrude { .. } => "surface_extrude",
+        FeatureKind::Unstitch { .. } => "unstitch",
+        FeatureKind::ReverseNormal { .. } => "reverse_normal",
         FeatureKind::Revolve { .. } => "revolve",
         FeatureKind::Fillet { .. } => "fillet",
         FeatureKind::Chamfer { .. } => "chamfer",