rename to localized content_id after translation

This pull request addresses plone/plone.app.multilingual/issues/27

Newly created translations are now marked with a flag, so that they can be recognised as new.
plone.multilingual registers a new event handler for `IObjectModifiedEvent`. If a newly added translation was modified, rename it based on its normalized title. (depending on some further checks)

Works both with AT and dexterity. Depends on my other pull request for PAM, where a new control-panel option to toggle this behaviour is added.

Please have a look at the code and check if you agree with my logic :-)