[Bug] Update upon children deletion
### Feature request / enhancement

Today layers in a group only update when their frames or positions change.
However, I ran into a case where I had a group of items:
![options](https://user-images.githubusercontent.com/6892625/37190264-c7e5a2ae-230d-11e8-9b5e-0211c6825148.png)

When I removed one of the items, I expected the group to update automatically but it didn't:
![options2](https://user-images.githubusercontent.com/6892625/37190296-ec3cb6ec-230d-11e8-82d7-920f4c23c4b7.png)

I had to intentionally move something by a pixel to make it refresh.
Can you please add the ability to update layers in a group upon deletion, or may be upon any change in the number of children? (Right now only increase in children number will trigger an update)

Thanks!