add an SDK classpath view

The view should list all SDK classpath entries (JAR name + maven GAV) and allow the user to download sources for a given JAR in order to have sources in Eclipse (sources will be downloaded to

/nxserver/sdk/sources).
The source download will be done from maven (may be local .m2 repository should be tried first)

This means we need a preference page where mave repositories may be configured - by default we ca add Nuxeo public repo + maven central repo.

This approach allows to use regular nuxeo distribs without having to include by default all jar sources inside)