Set the icon-name property as image hint instead of as app-icon

The icon specified in the FDO notify DBus call isn't an image that maybe is displayed in a notification but actually the app-icon to use. This isn't an issue if the notification server displays only one icon/image but if the app-icon is show in addition to the image in the image hint this causes obviously a problem, since the server expects the icon to be a app icon and not an image.

GNOME Shell will display the app-icon and image after gnome-shell!3103 (merged)