POC: Retrieve the latest version and compare it when compilation

Retrieves the latest version of GWTP from the maven API and compare it to the current version at compile-time. This is a POC and I don't expect to merge this branch yet.

Possible improvements:
- Make it a Pre-Linker: This way it will be automatically called at compilation without any other necessary interaction.
- Allow to disable the check with a property in the gwt module file
- Retrieving the current versions still feels too invasive to me
- ...