Added a todo.
Added GafferUI.MessageWidget, for displaying IECore MessageHandler output.

This is a refactoring of the insides of NotificationMessageHandler, with the following additions :

- access to the internal MultiLineTextWidget
- thread safety
- a method for appending exceptions as well as messages

Nothing in Gaffer itself is using a NotificationMessageHandler, and popups are a bit annoying generally, so maybe we could deprecate it now?
Added scrolling message output to OpDialogue.

Fixes #591.