Prepare for ICU update

- libicule and libiculx do not exist anymore in newer ICU versions
(harfbuzz replaces them), but we didn't actually use them, so remove
them from the build feature and from the package dependencies
- Add namespace usage marcos since the newer ICU packages put ICU things
in a namespace, making it easier to have multiple versions of ICU used
side by side.

No functional change intended, but this makes it possible to build the
code with either ICU 57 (for gcc2) or 66 (for other architectures).

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.