Add "Zoom" setting to the Help font settings

If the scrollWheelZooming is enabled, this
setting will get updated when using scroll wheel.
It will also be used when zooming in/out
by using keyboard shortcuts.

Fixes: QTCREATORBUG-25109
Fixes: QTCREATORBUG-25230
Fixes: QTCREATORBUG-23731
Change-Id: I9d22632b4c034ce236fa39dba074df4a2746ff84
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/help/helpviewer.h

@@ -54,9 +54,9 @@ public:
     virtual QFont viewerFont() const = 0;
     virtual void setViewerFont(const QFont &font) = 0;
 
-    virtual qreal scale() const = 0;
     virtual void setScale(qreal scale) = 0;
 
+    void setFontZoom(int percentage);
     void setScrollWheelZoomingEnabled(bool enabled);
     bool isScrollWheelZoomingEnabled() const;
 
@@ -87,9 +87,9 @@ public:
 
     void home();
 
-    virtual void scaleUp() = 0;
-    virtual void scaleDown() = 0;
-    virtual void resetScale() = 0;
+    void scaleUp();
+    void scaleDown();
+    void resetScale();
     virtual void copy() = 0;
     virtual void stop() = 0;
     virtual void forward() = 0;
@@ -117,6 +117,9 @@ protected:
     Actions m_visibleActions;
     bool m_scrollWheelZoomingEnabled = true;
     int m_loadOverrideStack = 0;
+private:
+    void incrementZoom(int steps);
+    void applyZoom(int percentage);
 };
 
 }   // namespace Internal