Help/litehtml: Support scaling

Scale the paint area for the HTML document

Change-Id: I75aa5f2bf057df7b2b160b2a005172ff88dbb2b7
Reviewed-by: hjk <hjk@qt.io>

src/plugins/help/litehtmlhelpviewer.h

@@ -77,6 +77,8 @@ public:
     void backward() override;
     void print(QPrinter *printer) override;
 
+    bool eventFilter(QObject *src, QEvent *e) override;
+
 private:
     void goForward(int count);
     void goBackward(int count);