Removed MessageWidget.textWidget() method.

The internal text widget should now be considered private. The currently displayed messages may be cleared using the new MessageWidget.clear() method.
Fixed MessageWidget crashes encountered in Maya.

After much stumbling around in the dark, I've thrown in the towel on trying to get block level visibility edits working, and resorted to a simpler approach where messages are stored independently of the text widget.
Removed deprecated MessageWidget.appendException() method.

It has been deprecated for a few versions now and is no longer used within Gaffer. The appendMessage() method is still used by NotificationMessageHandler so remains for now.
Merge pull request #767 from johnhaddon/messageWidget

MessageWidget fixes