Date formatting - make use of @ClientCallable that returns value

[vaadin-miki]

In Vaadin 14.2 methods annotated with @ClientCallable can return value. This means that custom date formatting and parsing can be done on the server, with completely custom methods.

This probably will remove the js generator in favour of a proper method that parses and formats based on DatePattern object.

[vaadin-miki]

This may be more difficult than it seems, as the @ClientCallable methods return a promise, whereas formatting / parsing methods on the client assume a value instead.

[vaadin-miki]

issues left to fully support server-side date formatting:

• the date picker changes its value to null before it gets back to the previous value; this happens when the overlay is opened and user clicks somewhere else than the overlay close button
• clicking "cancel" in the overlay does not work
• editing the date by typing it does not work

the last issue probably will never work, because of async, so in the worst case disabling the input might be the only option

[vaadin-miki]

closing this, as it will likely never get implemented - current DatePattern mechanism allows for most of the use cases, including month name as a localised string

if there ever comes a need for even wilder pattern generation, the existing code can be modified; point being with the current design of date picker it is not possible to fully do it server-side