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.

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.

Fix relying on order of static object destruction in Locale Kit.

* use only a single static object (MutableLocaleRoster) instead of
two, which avoids any problems if the order of static object
destruction would destroy RosterData before MutableLocaleRoster
* rename BPrivate::RosterData to BPrivate::LocaleRosterData and move
it into a header and implementation file of its own
This should hopefully fix problems encountered with a clang-compiled
Locale Kit.

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.

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.

Fix relying on order of static object destruction in Locale Kit.

* use only a single static object (MutableLocaleRoster) instead of
two, which avoids any problems if the order of static object
destruction would destroy RosterData before MutableLocaleRoster
* rename BPrivate::RosterData to BPrivate::LocaleRosterData and move
it into a header and implementation file of its own
This should hopefully fix problems encountered with a clang-compiled
Locale Kit.