Fix dropped callback not invoked

It seems that the dropped() callback is not invoked, please look at this plunker http://plnkr.co/edit/kspEDnmZj7FA4GvAWOHo.

The problem looks strange, it seems that after this invocation:
```
selectedElementScope.$dragInfo.apply(scope.$treeScope.copy);
```
The variable `selectedElementScope.$apply` becomes equal to `angular.noop` so the next time `$apply` is used it will practically do nothing, and the `dropped` callback won't be invoked:
```
selectedElementScope.$apply(function() {
  scope.$treeScope.$callbacks.dropped(dragInfoEventArgs);
});
```