Fix regression that broke FB2 input

Fixes #1186213 [ebook-viewer (portable 0.9.33) :: "Expected string or buffer" error](https://bugs.launchpad.net/calibre/+bug/1186213)
Allow right clicking on an author in Book Details

Allow right clicking on an author in Book Details to manage that author,
i.e. change the author name, sort value or link. Fixes #1186192 [Book Details Author right click - no options if file](https://bugs.launchpad.net/calibre/+bug/1186192)