Add property to enable middle click close for QTabWidget

Most of the modern web browsers allow closing tabs with a middle mouse click, making this feature somewhat natural user expected behaviour.

I think it would make sense to add a property to the QTabWidget which could be used enable/disable this behaviour. If my idea get positive feedback I can implement a patch.

I know it could be workarounded by creating a custom QTabBar and QTabWidget, but I still feel that many developers would benefit from it if it get integrated to Qt.