Add a method to clear the data to QAbstractItemModel

Now that QTBUG-45114 has been closed and the behaviour of setItemData is set in stone, it's clear QAbstractItemModel is lacking an interface to clear all the data from a single index.
I'm proposing to add a virtual bool clearData(const QModelIndex&) method to QAbstractItemModel so that the functionality can be implemented by the subclasses.