Only link to Qt5WebChannel if Qt5WebEngineWidgets available
Summary:
QWebChannel is only used if WebEngine is also available.
This converts scripting support into an optional feature which is only enabled if both dependencies are available.
Test Plan: Still builds.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: lbeltrame, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D27347