Date pickers should have an option to always accept short year as input

[vaadin-miki]

The use case is simple: the user wants to enter 1.8.20 and have it displayed as 1.8.2020 (or even 01.08.2020). Currently it is not possible, as short year is always displayed as two digits. The logic of figuring out the current century is already present in the client-side code (and also in the date pattern object on the server side), so it should be just a matter of a configuration option (when the date pattern has long year turned on, it could still accept short year as input).