Implement standard HasLabel interface

[mperktold]

There is a common interface com.vaadin.flow.component.HasLabel, which is now implemented by all major flow components.

See vaadin/flow#3241 and vaadin/flow-components#956.

The interface is compatible with the signatures of getLabel and setLabel. It's just a matter of declaring that MultiselectComboBox implements HasLabel.

It might even be possible to delete the current implementation, since HasLabel already defines default methods which are mostly equivalent. The only difference is that MultiselectComboBox .setLabel replaces null with an empty string, whereas HasLabel doesn't.