[FIX] playground: not all completed is deleted

Before this fix, not all completed tasks are deleted when clicking clear
completed button because we are looping to a mutated array. Looping thru
a copy of the mutated array fixes this problem.

tools/playground/samples.js

@@ -455,7 +455,7 @@ const actions = {
         todo.completed = !todo.completed;
     },
     clearCompleted({ state, dispatch }) {
-        for (let todo of state.todos) {
+        for (let todo of state.todos.slice()) {
             if (todo.completed) {
                 dispatch("removeTodo", todo.id);
             }