Add control over the default button to the OpDialogue.

This controls whether the OK or Cancel button is focussed by default when displaying the Op. The default value is as before, focussing the OK button, but the value can be controlled either by user data in the Op or by passing an alternative argument to the OpDialogue constructor.
Merge pull request #768 from johnhaddon/opDialogueFocus

Added control over the default button to the OpDialogue.