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