user_message_received
Description:
This signal is emitted when a UserMessage is received from 
      the WebKitWebView corresponding to web_page.
You can reply to the message using send_reply.
You can handle the user message asynchronously by calling @ref on message
       and returning true. If the last reference of message is removed and the message has been 
      replied, the operation in the WebKitWebView will finish with error webkit_user_message_unhandled_message
      .
Parameters:
| message | the UserMessage received | 
Returns:
| true if the message was handled, or false otherwise. |