Help viewer: Add option for scroll wheel zooming

Fixes: QTCREATORBUG-14154
Change-Id: Ia6c4aedb78954614477f0228ba82c4dc476a2525
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/help/webenginehelpviewer.h

@@ -53,6 +53,9 @@ class WebView : public QWebEngineView
 public:
     explicit WebView(WebEngineHelpViewer *viewer);
 
+    bool event(QEvent *ev) override;
+    bool eventFilter(QObject *src, QEvent *e) override;
+
 protected:
     void contextMenuEvent(QContextMenuEvent *event) override;