Metadata editor: Fix removing formatting for comments not removing background colors. Fixes #1258975 [Clear formatting button in comment editor does not clear span codes](https://bugs.launchpad.net/calibre/+bug/1258975)
Cover grid: Set text color based on the background color/image being used. Fixes #1258754 [[Enhancement] Font color for Cover grid field to show under the covers](https://bugs.launchpad.net/calibre/+bug/1258754)
Cover grid: Fix text showing 'None' for rating fields for books with no rating