Add open online documentation button to help viewer

Change-Id: I21685005709332b1201aaf08804399ecbd82bf7c
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/help/localhelpmanager.h

@@ -105,6 +105,9 @@ public:
 
     static void updateFilterModel();
 
+    static bool canOpenOnlineHelp(const QUrl &url);
+    static bool openOnlineHelp(const QUrl &url);
+
 signals:
     void filterIndexChanged(int index);
     void fallbackFontChanged(const QFont &font);