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/helpviewer.h

@@ -60,7 +60,6 @@ public:
     virtual qreal scale() const = 0;
 
     virtual QString title() const = 0;
-    virtual void setTitle(const QString &title) = 0;
 
     virtual QUrl source() const = 0;
     virtual void setSource(const QUrl &url) = 0;