Stuff split of sidebar / i18n

The Sidebar and I18n extension are given their own artifacts with this pull request.

Quoted from original spring-vaadin-stuff:
> Feel free to use the features listed in this README in your projects, but please
> consider this module slightly less stable (in terms of API) than the other modules in Vaadin4Spring.

These modules should have the possibility to evolve on their own.

The features present within the original *-stuff module are great and by giving them their own artifacts, people can choose to use them by simply including a dependency.