New implementation of using duplicate URLSegments

Now setting Translatable enforce_global_unique_urls to fals in config.
This makes LanguagePrefixTranslatable obsolete.

To prevent Translatable from still adding a locale suffix to new
translations, an onBeforeWrite() function is added. This should be a
temporary 'hack' I hope.
LanguagePrefixTranslatable no longer replaces Translatable

Check for Translatable instead
Updated the README
Fix: remove negation

This was a remnant of an earlier condition. Leave the URLSegment equal
to that of the original page if duplicate URLSegments are allowed.