Fix over-wide buttons in preferences
Bulk metadata edit: Allow setting comments and cloning covers

Bulk metadata edit: Allow setting the comments for all selected books
and also allow cloning the covers. Cloning covers means that the cover
of the first selected book will be set for all other selected books.
Fixes #1230040 [Bulk edit covers and comments [Enhancement]](https://bugs.launchpad.net/calibre/+bug/1230040)