Allow UIAlertController to get the press position when user long press the image.

Then a UIAlertController can be presented through these code in the imageViewerDidLongPress delegate.
```
[imageActionSheet.popoverPresentationController setSourceView:imageViewer.view];
[imageActionSheet.popoverPresentationController setSourceRect:rect];
[imageViewer presentViewController:imageActionSheet animated:YES completion:nil];
```