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>