Added MessageWidget.messageCount() method.

This returns the number of messages currently being displayed.
Made OpDialogue warning messages more prominent.

If an Op runs successfully, but outputs warnings or errors as messages, then these are noted in the completion message, and the details panel is expanded automatically. This is done even if the post execute behaviour states that the dialogue should close or return to the editing state.
Added OpDialogue.messageWidget() method.

This can be used to get access to the widget used to display messages from the op.
Added MessageWidget.forwardingMessageHandler() method.

This allows additional message handlers to be registered, so that messages can be passed on in addition to being captured and displayed by the widget.
Removed deprecated MessageWidget method usage from OpDialogue.

This also means that the result and error messages we append will make it to the MessageWidget.forwardingMessageHandler(), whereas with the deprecated methods they would not.
Merge pull request #734 from johnhaddon/opDialogue

Op warning/error display