Add subclassed dialog to wxUiTesting

Description:

This issue is both a bug and a feature. The base_class_name is not supported by the wxDialog generator -- that's the bug. The feature would be to add a custom class to wxUiTesting that derives from wxDialog and can therefore be used as a base_class_name. This class would bind to the wxEVT_INIT_DIALOG event and would grab the current title and append the current wxWidgets version to that title. That makes it marginally useful as well as a test case for supporting a subclasses wxWidget class in a different way then the normal widget subclasses.

This is related to KeyWorksRW/wxUiEditor#361.

Note that the property name has been changed to derived_class.