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/localhelpmanager.h

@@ -48,12 +48,18 @@ public:
     LocalHelpManager(QObject *parent = 0);
     ~LocalHelpManager();
 
+    static LocalHelpManager *instance();
+
     void setupGuiHelpEngine();
     void setEngineNeedsUpdate();
 
     static QHelpEngine& helpEngine();
     static BookmarkManager& bookmarkManager();
 
+    static QVariant engineFontSettings();
+
+    Q_INVOKABLE QByteArray helpData(const QUrl &url);
+
 private:
     bool m_guiNeedsSetup;
     bool m_needsCollectionFile;