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).

headers/os/locale/DateFormat.h

@@ -37,6 +37,14 @@ public:
 
 	static	const BDateFormat*	Default();
 
+			void				SetLanguage(const BLanguage& newLanguage);
+			void				SetFormattingConventions(
+									const BFormattingConventions& conventions);
+			status_t			GetDateFormat(BDateFormatStyle style,
+									BString& outFormat) const;
+			void				SetDateFormat(BDateFormatStyle style,
+									const BString& format);
+
 								// formatting
 
 			ssize_t				Format(char* string, const size_t maxSize,