[GFCarousel] timer of the autoPlay is not disposed

**Describe the bug**
In GFCarousel, inside dispose method, 
  ```
@override
  void dispose() {
    super.dispose();
    timer?.cancel();
  }
```
The problem here is that super.dispose is being called first and because of that timer?.cancel(); will never be executed.

**Expected behavior**
timer should be disposed before calling super.dispose();
