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. (reverted from commit ae1255461f82656d83a78a5555f0810620449eba)