Collapse V2, V3 and V4 classes into the top-level classes.

Qt has accumulated some classes such as QIconEngineV2, QStyleOptionViewItemV

etc for binary compatibility reasons.

Search for V<number> in assistant for the complete list.

The implementations of these classes should probably be collapsed into the 'topmost' class, making casting unnecessary to get the features (though leaving behind the empty classes or typedefs for source compatibility).