Add import option for maven repositories

[ctron]

In order to ease the workflow with maven, we should have an option to import an artifact from maven central, or another maven repository.

This would be a one time operation, compared to issue #1, which would regularly update the artifact.

[ctron]

There is a basic import infrastructure and workflow for importing from a maven repository or maven central.

The workflow needs a lot more style and customization options. But right now it works and can import main artifacts and their source attachments. Which in fact can create Eclipse source bundles.