Make BDateFormat mutable

* Add setters for the language and formatting conventions
* Add shortcut getter and setter for the date format
* Use those in the locale roster to make the BDateFormat actually use
the system preferred language and format.
* Applications can also use this to extract specific information from
the system format (eg. set date format to "LLLL" to extract month
names), or define specific formats more easily (eg. for parsing and
generating e-mail headers or HTTP cookies).