hack: message view: scroll back to top after load (work around WebView annoyance)

If the text of a message is longer than what fits one screen,
the WebView scrolles up and hides the header area of a new message.

Since there seems to be no way to prevent that without modifying
the framework itself, this might be the only solution:

Add a PictureListener and wait for the first complete
drawing of the view. Then, scroll back to top and unregister
the listener.