Add prop onSortOver on SortableContainer

[SamyPesse]

This PR adds a prop onSortOver. The goal of this prop is for the parent component to be notified when the user is dragging over another item before it drops it.

It follows the convention of the other props: named as onSortX with same named arguments.

Example:

Let's say we want to show an alert message at the bottom of the page if we are hovering a specific item:

onSortOver = ({ newIndex, oldIndex }) => {
  const { items } = this.state;
  const item = newIndex != null ? items[newIndex] : null;

  this.setState({
    showAlert: item && item.someProperty
  });
});

render() {
   return (
       <div>
           <SortableList
               items={this.state.items}
               onSortEnd={this.onSortEnd}
               onSortOver={this.onSortOver}
           />
           {this.state.showAlert && <div>Some message !</div>}
       </div>
   );
}

[clauderic]

This is great, thanks for contributing 🙏

Sorry for the late review 😅