Added events to WizardPage so it can cancel a page change. Fixed #7.

This add the events of `onNext()` and `onPrev()` that will be called before going to the next page (or before finishing) and before going back so the change can be cancel if data isn't valid.

The method are called before the settings are updated so in settings we have valid settings all the time.

Also fixed #8, UpdateSettings not called before finish.