Help: Allow open online documentation for CMake

Qt Creator opens by default the offline documentation of CMake.
But the user can click on the "Globe" to go to the online version
of the documentation.

Change-Id: I0b3a6bceb13784b232b539f1c04bd09aa3a11034
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/helpmanager.cpp

@@ -103,5 +103,11 @@ void setBlockedDocumentation(const QStringList &fileNames)
         m_instance->setBlockedDocumentation(fileNames);
 }
 
+void addOnlineHelpHandler(const OnlineHelpHandler &handler)
+{
+    if (checkInstance())
+        m_instance->addOnlineHelpHandler(handler);
+}
+
 } // HelpManager
 } // Core