Added two new image specific properties 'tileCountHorizontal' and 'tileCountVertical' to class Format. DashManifestParser adjusted to parse these values from an EssentialProperty tag of an Image AdaptationSet. With this change, DashManifest does not have to do any parsing inside the new getThumbnailDescriptions() method. Moreover, both classes ThumbnailDescription and ThumbnailProvider adjusted to use these two properties / naming scheme accordingly.
