Better labels for ContentTypeRadioInputs.

Every once in a while, I create a widget, the content type gets stored in the database and then I go to change the `verbose_name` of that widget, but the ContentType label is never updated in the database. This patch makes sure to use the current `verbose_name` of the model class. This should also enable the internationalization of of the ContentTypeRadioInputs, although I did not write a test for that.