BMessageFormat: Rename to BStringFormat.

The ICU class is named MessageFormat, but on Haiku, it sounds too much
like something related to BMessage (which it isn't in the slightest)
and not part of t

BMessageFormat: Rename to BStringFormat.

The ICU class is named MessageFormat, but on Haiku, it sounds too much
like something related to BMessage (which it isn't in the slightest)
and not part of the Locale system. It works almost entirely with BStrings,
so naming it BStringFormat makes much more sense.

OK'ed by PulkoMandy and Humdinger.

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