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/bpy_app.c

@@ -36,6 +36,10 @@
 #include "bpy_app_ffmpeg.h"
 #include "bpy_app_build_options.h"
 
+#ifdef WITH_INTERNATIONAL
+#  include "bpy_app_translations.h"
+#endif
+
 #include "bpy_app_handlers.h"
 #include "bpy_driver.h"
 
@@ -86,7 +90,10 @@ static PyStructSequence_Field app_info_fields[] = {
 	{(char *)"ffmpeg", (char *)"FFmpeg library information backend"},
 	{(char *)"build_options", (char *)"A set containing most important enabled optional build features"},
 	{(char *)"handlers", (char *)"Application handler callbacks"},
-	{NULL}
+#ifdef WITH_INTERNATIONAL
+	{(char *)"translations", (char *)"Application and addons internationalization API"},
+#endif
+	{NULL},
 };
 
 static PyStructSequence_Desc app_info_desc = {
@@ -152,6 +159,9 @@ static PyObject *make_app_info(void)
 	SetObjItem(BPY_app_ffmpeg_struct());
 	SetObjItem(BPY_app_build_options_struct());
 	SetObjItem(BPY_app_handlers_struct());
+#ifdef WITH_INTERNATIONAL
+	SetObjItem(BPY_app_translations_struct());
+#endif
 
 #undef SetIntItem
 #undef SetStrItem