Improve mechanism of cache of image in ImageView.

The mechanism of cache (saving and loading) is defined by the cache policy.
We have 2 levels of cache like: memory cache and disk cache.
When receiving a request of loading image with URL , the ImageView will try the actions followings:
- Loading image in cache depending on the policy.
- Else loading image in download directory.
- Else downloading with this url.

See QCAImageView and QCImageCache.