Add i18n placeholder upgrade.

Inspired by https://github.com/cakephp/cakephp/issues/4321 I tried to automate the upgrade of placeholders in __() translation functions.

I do not quite like the numeric indexing though.
If you have like `{0} {1} {2}` and you want to add one placeholder at the beginning, you would have to reindex all other ones, as well. But I guess that is not occuring too often, so it's fine.
I don't think it does need to be public - as it can only be used internally from _process().
Unfortunately we don't have any tests yet. But I can sure add one for this.