Help: Provide native WebView backend on Mac

Run Qt Creator with environment variable QTC_HELPVIEWER_BACKEND to
* 'native' to get the WebView based one on Mac
* 'textbrowser' to get the QTextBrowser based one

Defaults to use QWebView if QtWebKit is available, or QTextBrowser if
not, like before.

Change-Id: If0660782b18ff3d89301fa7bcaf4e2e2fb69627d
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/help/qtwebkithelpviewer.h

@@ -59,7 +59,6 @@ public:
     qreal scale() const;
 
     QString title() const;
-    void setTitle(const QString &title);
 
     QUrl source() const;
     void setSource(const QUrl &url);