Python i18n API. Many thanks to Campbell and Brecht for the reviews and suggestions!

This commit adds:
* A new bpy.app.translations module giving some info about locales/translation stuff (current active locale, all locales currently known by blender, all translation contexts currently defined, etc.).

* The ability for addons to feature translations, using the (un)register functions of above module.

* Also cleans up "translate py string when storing into RNA prop" by removing "PROP_TRANSLATE" string's subtype, and adding a PROP_STRING_PY_TRANSLATE flag instead (this way it is no more exposed to python...).

Addon translations work with py dictionaries: each addon features a dict {lang: {(context, message): translation, ...}, ...}, which is registered when the addon is enabled (and unregistered when disabled). 

Then, when a key (context, message) is not found in regular mo catalog, a cache dict for current locale is built from all registered addon translations, and key is searched in it.

Note: currently addons writers have to do all the work by hand, will add something (probably extend "edit translation" addon) to automate messages extraction from addons soon(ish)! To get a look to expected behavior from addons, have a look at render_copy_settings/__init__.py and render_copy_settings/translations.py (rather stupid example currently, but...). Once we have a complete process, I'll also update relevant wiki pages.

source/blender/python/intern/CMakeLists.txt

@@ -49,6 +49,7 @@ set(SRC
 	bpy_app_ffmpeg.c
 	bpy_app_build_options.c
 	bpy_app_handlers.c
+	bpy_app_translations.c
 	bpy_driver.c
 	bpy_interface.c
 	bpy_interface_atexit.c
@@ -72,6 +73,7 @@ set(SRC
 	bpy_app_ffmpeg.h
 	bpy_app_build_options.h
 	bpy_app_handlers.h
+	bpy_app_translations.h
 	bpy_driver.h
 	bpy_intern_string.h
 	bpy_library.h