longtap gesture

If you need access to the view for zooming from the cell to the full image instead of exposing the specific NFTPhotoAssetCell I think it would be better to simply expose the UIView. Example signature:

```
- (void) assetsGroupViewController:(NFTAssetsGroupViewController *) assetsGroupViewController didLongTouchAsset:(ALAsset *)asset inView:(UIView *)view;
```

The NFTImagePicker delegate could be updated to support a similar signature to send to the user of the image picker.