Avoid dom error when trying to remove an already removed Node

In case the `spin.el` is removed from DOM because of a parent element change, when calling `spin.remove()` throws the following:

```js
Uncaught NotFoundError: An attempt was made to reference a Node in a context where it does not exist
```

I added a `removed` listener with `component/removed` to change the `appended` flag to `false`.