Move date formatting from BLocale to BDateFormat

* There is a little code duplication. This will be moved to BFormat once
the time and datetime formatting is also moved out of BLocale
* The way to create a BDateFormat from a BLocale is still open for
discussion. I'm undecided between making BDateFormat a member of
BLocale, or adding a BDateFormat(const BLocale&) constructor.
* Adjust all users of the API.