Add quality to GrabbedImage class

[javidsho]

We've already defined image size for grabbed images. An estimated image quality is also necessary to let library users choose the image with most or least quality or resolution. ImageQuality shall be defined as double to let grabbers proportionally define the quality or resolution of the image. For example, YouTube provides images with qualities hq, lq etc. in which case we don't know the size of the images and YouTubeGrabber does not provide any information about the difference of GrabbedImages. However thanks to this new property defined, the metadata of those images are now different.