Fix building akregator with QTWEBENGINE_EXPERIMENTAL_OPTION

Summary:
0d0a5445 added the definition of the
method "WebEngineFrame::slotExpandUrlInFrame(int frameId)" but not
its declaration.
Also expand (the accidental, I suppose) "slotE" to its full name
"slotExpandUrl".

Reviewers: kde-pim, mlaurent

Reviewed By: mlaurent

Projects: #kde_pim

Differential Revision: https://phabricator.kde.org/D1083