If false, will make this widget's descendants unfocusable.
Defaults to true. Does not affect focusability of this node (just its descendants): for that, use canRequestFocus.
If any descendants are focused when this is set to false, they will be
descendantsAreFocusable is set to true again, they will
not be refocused, although they will be able to accept focus again.
Does not affect the value of canRequestFocus on the descendants.
final bool descendantsAreFocusable