Special boolean property which allows for atomic focus change.
Focus change means defocusing the old focus owner and focusing a new
one. With a usual property, defocusing the old node fires the value
changed event and user code can react with something that breaks
focusability of the new node, or even remove the new node from the scene.
This leads to various error states. This property allows for setting
the state without firing the event. The focus change first sets both
properties and then fires both events. This makes the focus change look
like an atomic operation - when the old node is notified to loose focus,
the new node is already focused.