Support notifying input context for focus changes inside non-QWidget components

QInputContext doesn't see inside QGraphicsView or any other non-QWidget. Biggest problem there is probably that we don't know when the focus changes inside the graphicsview. This would be nice to know if we want to reset some virtual keyboard state, e.g. close some extra character views. As a workaround on Meegotouch we now ourselves call QInputContext::setFocusWidget() with currently focus QWidget.

This should be either done on Qt side for all appropriate views or Qt should provide some other mechanism for such notifications.