Improve quality of images manipulated by calibre. In particular improve ...

...images dropped or pasted into edit metadata. Note: Qt's default compression quality is 75, which is very low. Perhaps calibre's default doesn't need to be as high as 90, but it should be higher than 75. Perhaps this is a good place for a tweak.

BTW: when an image is dropped on edit metadata single, even with the quality set to 100 the resulting cover.jpg is smaller than the original. Why this should be is a mystery. I suspect that Qt's pixmap processing is doing some extra compression. At any rate, the images are much better now than they used to be.