Make BDateFormat inherit from BFormat again

* Move relevant parts up into BFormat so other format classes can use
those
* Adjust BDurationFormat and BTimeUnitFormat for the changes
* Remove the "default" date format, it is better to keep only a default
locale and let applications create B*Formats from it as needed.
* Creating a B*Format without arguments to the constructor now
configures it for the default locale, which allows for easy use in
standard cases (formatting something with the current language and
format)
* Creating a B*Format is potentially an expansive operation, it is
advised to keep the instance around and reuse it whenever possible.
However it must be "refreshed" when the locale changes, for apps which
supports that, since it keeps a copy of the language and formatting
convention, rather than a pointer to the locale as it did before.
Add tests for BDurationFormat and BTimeUnitFormat.
Adjust all BDateFormat callers again.