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.
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