onMouseDown doesn't have the event parameter passed in correctly

This is the current onMouseDown event that is bound.

```
case 'onMouseDown':
{
  _this.editor.on('mousedown', function(cm) {
    _this.props.onMouseDown(_this.editor, event);
  });
  break;
}
```

The callback function doesn't have an event variable that is passed in - which causes **ReferenceError: event is not defined** in some browsers like Firefox.