* New menu option and filemanager filter to hide finished books #7158
The default behavior is to display the finished books (no change on
upgrade). For consistency with the two similar options, it represented
by a checkbox "Show hidden books" that is checked by default.
The implementation is straightforward, meaning that, when the option is
unchecked, each file will require a call to `filemanagerutil.getStatus`
that checks its status.
For clarity, the code uses the "finished books" expression because the
condition is relevant to the *book* metadata, while the other settings
are about *file* attributes.