Update BigRedButton usage sites, add callback note

src/widgets/machines.rs

@@ -42,7 +42,11 @@ impl CuttingMachine {
           BackgroundColor(DARK_GRAY.into()),
           Pickable::default(),
         ))
-        .with_children(|cmds| BigRedButton::spawn_big_red_button(cmds, "CUT"));
+        .with_children(|cmds| {
+          let _button_cmds = cmds.spawn(BigRedButton::bundle("CUT"));
+          // TODO: Attach on-press observer so this machine can do something
+          // in response to that button being pressed
+        });
     });
   }
 }
@@ -82,7 +86,10 @@ impl RotatingMachine {
           BackgroundColor(DARK_GRAY.into()),
           Pickable::default(),
         ))
-        .with_children(|cmds| BigRedButton::spawn_big_red_button(cmds, "TURN"));
+        .with_children(|cmds| {
+          let button_cmds = cmds.spawn(BigRedButton::bundle("TURN"));
+          // TODO: Attach on-press observer to the button.
+        });
     });
   }
 }