Added composite repo and removed the sdks

[keinhaar]

This is intended for Release 4.2. It add's composite repo files and removes the sdk's, which can be build separately by the gwt-sdk-plugins project. The Update Site should look like this:

UPDATE_URL/
    compositeContent.xml
    compositeArtifacts.xml
    gwt-eclipse-plugin/
        content.xml
        artifacts.xml
        plugins/
        features/
        ...
    gwt-sdk-plugins/
        content.xml
        artifacts.xml
        plugins/
        features/
        ...

I think, we should also have a latest folder for the latest version, because otherwise we always have to update documentation pages and the marketplace entry. Another thing to think about: Should we move the Update Site to our own documentation page? Otherwise we depend on the GWT Project itself to update the repo.