Rename translations based on their title

Addresses #27

The actual renaming happens in plone.multilingual, see plone/plone.multilingual/pull/3

This pull request adds a control-panel option to toggle the renaming, since there might be use cases where the ids of objects should not change based on their titles.

I amended 2 existing tests (form.txt and language_independent.txt) to not make assumptions about the ids of translations (not relevant for those tests).

I added a new test (test_rename_translations.txt) that explicitly tests the renaming.