update modal menu prompt

ezgui/src/input.rs

@@ -296,6 +296,7 @@ impl UserInput {
                 panic!("set_mode called on both {} and {}", existing_mode, mode);
             }
             menu.mark_all_inactive();
+            menu.change_prompt(prompt);
         } else {
             if let Some(ref m) = self.modal_state.modes.get(mode) {
                 let mut menu = Menu::new(

ezgui/src/menu.rs

@@ -210,4 +210,10 @@ impl<T: Clone> Menu<T> {
             *active = false;
         }
     }
+
+    // Assume that this doesn't vastly affect width.
+    pub fn change_prompt(&mut self, prompt: String) {
+        assert!(self.prompt.is_some());
+        self.prompt = Some(prompt);
+    }
 }