Fix Ctrl+mouse_scroll zoom of documentation view

In some Qt version between 5.9 and 5.14 QWebEngineView stopped
propagating mouse wheel events to its parent, which broke the only GUI
way to scale documentation view. Users could still set their preferred
documentation view zoom levels in the KDevelop configuration file.

This fix benefits KDevelop built with Qt WebKit too - Ctrl+mouse_scroll
not only scaled documentation view, but at the same time scrolled it as
QWebView ignored the Ctrl modifier key.