Better support usage of user data in QComboBox

QComboBox has a feature that allows supplying user data that is stored in QVariants. This user data is very useful since it directly links any processable and meaningful value to the user selection. This allows intuitive, compact and easy to maintain code. Unfortunately QComboBox does not provide a signal that emits the user value (it only provides text and index based signals).

This makes the user-data feature a bit of a pain on the reactive side. Also the user data feature can not be used at all if the QComboBox is constructed as part of an anonymous widget (without a derived class for the parent widget) because no reference to the QComboBox exists that would allow index->userdata lookup.

Please add new signals that emit the user data as a QVariant.