make UI optional in the PageConfigurator interface (deprecate and create another interface)

[vlukashov]

The V14 PageConfigurator interface should be deprecated and replaced by a new AppShellConfigurator interface. The difference between the two interfaces is that PageConfigurator receives a stateful InitialPageSetting parameter, whereas AppShellConfigurator receives a stateless-by-default AppShellSettings parameter.

Statefulness of InitialPageSettings means that it assumes existence of a server-side session: a UI instance, a server-side navigation event, and UI-bound push config, loading indicator config, and error message config.

Stetelessness-by-defualt of AppShellSettings means that it does not assume existence of any server-side session, but optionally allows it. The methods that involve the server-side state are all wrapped with an Optional<> (e.g. public Optional<UI> getUi()).

The new AppShellConfigurator interface would be either implemented by the MainLayout class (if the app has a server-side main layout) or by the AppShell class (if the app has a client-side main layout).

An instance of the app shell class is created during the VaadinService initialization, and the configureAppShell() method is called every time when IndexHtmlRequestHandler generates an index.html page.

AppShellConfigurator may be implemented by an AppShell instance (one per Vaadin Service), and also by a MainLayout (one per UI). What should happen in this case?

• shell-level runs first
• ui-level runs after that