Fix webenginemessageviewer with Qt 5.11

Summary:
QtWebEngine 5.11 can intercept more messages include local and data ones that
are essential to the working of the message viewer.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D11122