enable resetting doc browser zoom level even when zooming with mouse wheel * documentation.cc (documentation_browser::wheelEvent): reimplement mouse wheel event and, in case of active control modifier, call own zooming function which are keeping track of the zoom level * documentation.h: new protected method for reimplemented wheel event

--- a/libgui/src/documentation.h	Tue Oct 30 14:34:57 2018 -0700
+++ b/libgui/src/documentation.h	Tue Oct 30 22:50:40 2018 +0100
@@ -59,6 +59,10 @@
     void zoom_normal (void);
     //!@}
 
+  protected:
+
+     void wheelEvent (QWheelEvent *we);
+
   private:
 
     QHelpEngine *m_help_engine;