app.alert timeout bug

[barisusakli]

If an app.alert is made with timeout and alert_id values set, and another timeout is made with the same alert_id, the old setTimeout(); is not cleared to make way for the new timeout, so the alert fades away prematurely.

Reproduction steps

1. `app.alert({ alert_id: 'foo', timeout: 10000 });
2. Wait five seconds.
3. `app.alert({ alert_id: 'foo', timeout: 10000 });
4. Alert disappears after another five seconds, instead of the expected ten.