Add new protocol for 7z archives

Summary:
This patch adds a new protocol, sevenz, for 7z archives by utilizing K7Zip. Previously, users have to install Krusader to use its krarc protocol to navigate a 7z archive using Dolphin.

Depends on D23780
Depends on D23490

Test Plan:
Compile and install KIO with D23490 and D23780 applied.

(If Krusader is installed, change krarc to sevenz in the URL bar if necessary.)

Test using Dolphin:
Enable "Open archives as folder" in Dolphin, then open and browse a 7z archive using the sevenz protocol.

Test using Gwenview:
View a 7z archive containing photos. Browse and view images inside the archive.

Reviewers: meven, ngraham, dfaure, pino, cfeck

Reviewed By: meven, ngraham

Subscribers: ltoscano, meven, ngraham, kfm-devel, kde-frameworks-devel

Tags: Frameworks, Dolphin

Differential Revision: https://phabricator.kde.org/D23730