Add unarchiver support for cbr files

Summary:
FEATURE: 339370
When no unrar installed, Okular now utilizes the program unar for extracting the archive and lsar for listing the content in the archive.
lsar is related and comes with a unar installation.

Subscribers: aacid, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D15691