Thin Extrude: walls of a set thickness along sketch curves (#721) (#991)

Thin Extrude builds walls of a set thickness along sketch curves, open chains and closed loops, with the wall centred, inside, outside or on both sides at two thicknesses. Saved under format 20 (thin on an Extrude); an extrude without thin saves as before. Closes #721.

crates/ui-egui/src/lib.rs

@@ -125,6 +125,8 @@ mod sketch_text_move;
 pub mod sketch_tools;
 pub mod stand_up;
 pub mod theme;
+#[cfg(test)]
+mod thin_extrude_tests;
 pub mod timeline;
 pub mod timeline_left;
 pub mod titlebar;