v1.1.1

- Implemented ability to insert clips into a stacked track at a given index e.g. `track.add(new Clip(), 2)`
- Ensured clips within stacked tracks can be split without the track being reordered

**Example**
```
const composition = new core.Compostion();
const track = composition.createTrack('base').stacked();

await track.add(new core.Clip({ name: 'foo' });
await track.add(new core.Clip({ name: 'bar' });

// now add a clip in between
await track.add(new core.Clip({ name: 'pong' }), 1);


console.log(track.clips[0].name); // foo
console.log(track.clips[1].name); // pong
console.log(track.clips[2].name); // bar
```