Refactor translation code out of blenfont

- Add blentranslation `BLT_*` module.
- moved & split `BLF_translation.h` into (`BLT_translation.h`, `BLT_lang.h`).
- moved `BLF_*_unifont` functions from `blf_translation.c` to new source file `blf_font_i18n.c`.

source/blender/python/SConscript

@@ -36,6 +36,7 @@ incs = [
     env['BF_GLEW_INC'],
     '#/intern/glew-mx',
     '#/intern/cycles/blender',
+    '../blentranslation',
     '../blenfont',
     '../blenkernel',
     '../blenlib',