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_props.c

@@ -74,13 +74,15 @@ static EnumPropertyItem property_flag_enum_items[] = {
 	{0, NULL, 0, NULL, NULL}};
 
 /* subtypes */
+/* XXX Keep in sync with rna_rna.c's property_subtype_items ???
+ *     Currently it is not...
+ */
 static EnumPropertyItem property_subtype_string_items[] = {
 	{PROP_FILEPATH, "FILE_PATH", 0, "File Path", ""},
 	{PROP_DIRPATH, "DIR_PATH", 0, "Directory Path", ""},
 	{PROP_FILENAME, "FILE_NAME", 0, "Filename", ""},
 	{PROP_BYTESTRING, "BYTE_STRING", 0, "Byte String", ""},
-	{PROP_TRANSLATE, "TRANSLATE", 0, "Translate", ""},
-	{PROP_PASSWORD, "PASSWORD", 0, "Password", 0},
+	{PROP_PASSWORD, "PASSWORD", 0, "Password", "A string that is displayed hidden ('********')"},
 
 	{PROP_NONE, "NONE", 0, "None", ""},
 	{0, NULL, 0, NULL, NULL}};