Defines how the floating label should behave.
When FloatingLabelBehavior.auto the label will float to the top only when the field is focused or has some text content, otherwise it will appear in the field in place of the content.
When FloatingLabelBehavior.always the label will always float at the top of the field above the content.
When FloatingLabelBehavior.never the label will always appear in an empty field in place of the content.
Defaults to FloatingLabelBehavior.auto.
final FloatingLabelBehavior floatingLabelBehavior;