add frontend folder structure

[OlliTietavainenVaadin]

Making new template-based add-ons would be easier if there was a frontend folder already present in the file structure (perhaps with a helpful readme file like in the base starter).

This should go to resources/META-INF/resources/frontend

For development time, the Jetty plugin configuration should also have <resourceBase>${project.basedir}/src/main/resources/META-INF/resources</resourceBase>
in the webAppConfig

[eriklumme]

I believe if multiple resource bases are set, it should be set inside a resourceBases:

<resourceBases>
    <resourceBase>a</resourceBase>
    <resourceBase>b</resourceBase>
</resourceBases>

Otherwise the second overrides the first.