Add a new port www/qt5-webchannel

Qt-5.4 introduced a new component: Qt WebChannel

The Qt WebChannel module provides a library for seamless integration of C++ and
QML applications with HTML/JavaScript clients. Any QObject can be published to
remote clients, where its public API becomes available.

https://doc-snapshots.qt.io/qt5-5.4/qtwebchannel-index.html

Approved by: makc (mentor)
Submitted by: Tobias Berner <tcberner@gmail.com> via area51 commit