Update the `required` attribute of both input and control input.

The `required` property needs to be activated on the control input
for the error to be displayed at the right place. `required` also
needs to be temporarily deactivated on the input since the input is
hidden and can't show errors.
function() {
		if (!this.isRequired) return false;

		var invalid = !this.items.length;

		this.isInvalid = invalid;
		this.$control_input.prop('required', invalid);
		this.$input.prop('required', !invalid);
	}