Support enabling/disabling keyboard search in views

Curently, QAbstractItemView's keyboard search feature is enabled by default, with no option to disable it apart from subclassing the view and reimplement keyboardSearch() to do nothing.

This is a problem, for example if a view has AllEditTriggers, but not all items are editable. If a non-editable cell is current, and the user presses a key, the keyboard search is initiated, which may make an editable item current, and a subsequent key press starts editing the item.

It would be nice if Qt supported setting whether keyboard searching should be enabled or not.