Fix StackView focus handling

Make StackView a FocusScope so inner focus changes don't have
undesired side effects in other parts of the application.

Task-number: QTBUG-42506
Change-Id: I0d901083196cf90f236a24e1dcd2e91bd9f320db
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@theqtcompany.com>