Python SDK to support client applications

[kbairak]

1. Initialization logic for TxNative and CDSHandler

See my blog post for the reasoning behind this.

2. Language management:

1. TxNative has the following properties:

   • source_language_code (recommended, is set with the source_language
     kwarg to setup)
   • current_language_code (can be None, is set with the current_language
     kwarg to setup, or with set_current_language)
   • hardcoded_language_codes (can be None, is )
   • remote_languages (is populated automatically)

2. fetch_languages will fetch the list of available languages from Transifex
   and save them in the remote_languages attribute. It will then return the
   intersection of the languages in remote_languages and
   hardcoded_language_codes if the latter has been set or all the remote
   languages otherwise. Fetching from the CDS only happens the first time.

3. fetch_translations accepts an optional language_code argument. If set,
   it will only fetch the specified language. This will also fetch the remote
   languages lazily to check that the specified language is available.

4. set_current_language will lazily fetch the remote languages. It will then
   lazily fetch the translations for the specified language and finally set the
   current_language_code attribute to the specified language.

5. The language_code argument to translate is now optional. If not set, the
   current_language_code attribute will be used.

3. Event emitter

Same signatures as with the javascript SDK. You can call tx.on(EVENT_LABEL, callback) and then interesting things will happen when the current language is changed or when the translations are fetched

4. urwid SDK

Simple SDK to allow urwid applications to behave somewhat like react applications. See here for a demo