chat: show images in attachments: - if an image is received from somebody in roster, it is automatically displayed (we display only for people in roster to avoid IP address leak) - encrypted files are decrypted and stored in cache before being displayed - GIFs image are shown as attachment because they are badly handle in Kivy (images frequencies is not handled correctly, and memory consumption explode). Instead, a click on it will open the GIF in the appropriate software of the platform. - a click on an attachment image will open it in the gallery