Implement the reviewed UI icon system and durable review workflow (#853)

Thanks @Nathan-Dane.

crates/engine/src/cmd/clipboard.rs

@@ -12,8 +12,9 @@ use crate::params::{bad, string_list, vec3};
 use crate::{Result, Session};
 
 pub static COMMANDS: &[CommandSpec] = &[
-    CommandSpec::new("timeline.copy", "Copy Features", copy).noundo().params("features: [ids or names] (in timeline order on paste)"),
+    CommandSpec::new("timeline.copy", "Copy Features", copy).icon("copy").noundo().params("features: [ids or names] (in timeline order on paste)"),
     CommandSpec::new("timeline.paste", "Paste Features", paste)
+        .icon("paste")
         .params("translate?: [x,y,z] (moves the copies' geometry); the copies go at the marker"),
 ];