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