Throw exception if using CommandButtons and CustomButtons

Using command links together with custom buttons is not supported by the native TaskDialog implementation. If not prevented by this wrapper, doing so leads to unexpected behaviour.
Common buttons localisation; Cancel-only button

Added static properties to the TaskDialog class that allow providing localised texts for the common buttons like Cancel or Retry. Set them once before using a TaskDialog. If null, the English default will be used as before.

Also, using only the Cancel button of the common buttons is now possible.