[list-box] Add validation APIs

[pekam]

Are you planning to implement required, invalid, errorMessage etc. for <vaadin-list-box>?

We would especially need the required-property for our Flow API of the element.

[tomivirkki]

Weren't planning on including these APIs in the first release. The current API is a subset of <paper-listbox>'s API which also doesn't include value or these other form field -properties you mentioned.

[brp-rasmus]

Hi!
We also need this functionality, especially the required-property.

[cantaylancapraz]

Any update?

[nSharifA]

+1

[downdrown]

+1

[amagnolo]

+1

[pichlerm]

+1

[cfacianoTh]

+1