Help: Do not pass zoom factor to constructor

Introduce HelpViewer::setScale instead

Change-Id: I9d50483bc1fa4de89de53db40eda348379bd0ae9
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/help/helpviewer.h

@@ -55,6 +55,7 @@ public:
     virtual void setViewerFont(const QFont &font) = 0;
 
     virtual qreal scale() const = 0;
+    virtual void setScale(qreal scale) = 0;
 
     virtual QString title() const = 0;