Add option of connecting to the vnc server through a ssh tunnel
Summary: Introduces an optional dependency on libssh
Test Plan: Connect to VNC servers both via SSH tunnel and via regular connection
Reviewers: dfaure, uwolfer
Reviewed By: dfaure, uwolfer
Subscribers: dfaure, acelankao, uwolfer, KDE Applications
Differential Revision: https://phabricator.kde.org/D13659