Try to make people notice that they are missing @Theme

[pleku]

When creating an app based on a starter and adding a new navigation target structure, if I forget to add the Theme annotation, I get a very ugly UI and don't know what to do without reading docs

Since the Vaadin elements are not really meant to be used as directly the un-themed versions from /src/ folder, we should make it as clear as possible to tell the users that they are missing the @Theme annotation from the page they opened.

One option was to apply Lumo by default, but we discarded this because it has some unfortunate side-effects for advanced users building their own themes or not using Lumo or our components, and would force us to try to detect for both the Lumo resources presence and whether @Theme is present or not.

For now let's just add logging with warning level to the server console when no @Theme is present and we are serving the unthemed versions of Vaadin Components, since those are not meant to be used.