Overlay should not close when clearing the field using the clear button

[jouni]

The same issue as for combo-box (vaadin/vaadin-combo-box#523)

If the overlay was open when the user clears the field using the clear button, the overlay should not close.

If the user clears the field using the clear button we should open the overlay even if it wasn’t open before, because the field is then focused implicitly using the mouse and that should open the overlay.