Add support for cb7 comic book archives

Test Plan: Open a cb7 file (e.g. [[ https://bugs.kde.org/attachment.cgi?id=122002 | Attachment 122002 ]] from [[ https://bugs.kde.org/show_bug.cgi?id=410696 | Bug 410696 ]]) using Okular and see if it works.

Reviewers: #okular, aacid

Subscribers: broulik, pino, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D23037