Add QComboBox::currentData

Convenience function that returns user data for the current item,
analogous to currentText().

This avoids to having to write the cumbersome

  QVariant variant = comboBox->itemData(comboBox->currentIndex());

(It's quite common to put strings as text and the corresponding
enum values as user data.)

Change-Id: I8c7632c647c5583d18e4e22703aeb4447d73162d
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>