Add new protocol for 7z archives

Summary:
D23730 adds the sevenz protocol for 7z archives by utilizing K7Zip, so this patch is needed for proper URL navigation.

See also: D20424

Reviewers: Frameworks, cfeck, pino, dfaure, meven, ngraham

Reviewed By: ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23780