Fix the fix for #391300 to require only QtWebEngine 5.10, not Qt 5.10

QtWebEngine can be newer than the rest of Qt (e.g., on Fedora 27).

This fixes the version check in WebView::savePageAs to use a macro
defined in config.h.cmake based on the version of QtWebEngineWidgets
rather than relying on QT_VERSION, which is the version of QtCore.

Reviewed By: drosca
Differential Revision: https://phabricator.kde.org/D12833
CCBUG: 391300