It is about default behavior for QQuickWindow, when there is no any
activeFocusItem in the whole window, Tab/BackTab key could help you
to focus the item in the tab focus chain.

Change-Id: Iccd770794e2f99a9a74c3a11f0ed278264a02d9e
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>