Make it possible to use multiple connectors at once

Based on requests in http://drupal.org/node/321490 and http://drupal.org/node/204171 it is evident that people might need to use multiple connectors. One to translate local packages for example and another to upload .po files for modules hosted or developed elsewhere such as Acquia modules or modules from http://www.netrift.com/ as pointed out in http://drupal.org/node/321490

This made it important to let connector modules co-exist, and "own" specific projects. This lets connectors only rescan projects they handle, while leave others alone for the other connectors to manage. With this patch a project has an associated connector, and two connectors are not allowed to own the same project. So you can mix different connectors being used on your site.

The update tries to do its best to specify the right connector for existing modules. Those using a custom connector will need to update their database manually.

Committed to 6.x.