Fix translations in non standard prefix

This drops a lot of code at the same time. The code in
__init__ was:

* Using a SUGAR_PREFIX environment variable that is not
defined anywhere
* Hardcoding prefix to /usr
* Setting up the sugar-base domain which doesn't exist anymore

The i18n module code was overcomplex because it was thought
for language packs which we don't support anymore, it was
not handling correctly locales with a country prefix, it
was hard coding the python prefix and it was not sorting
the directories as it intended too anyway.

The logic is very simple now. Use the locale directory in
the same prefix sugar-toolkit-gtk3 was installed, unless
SUGAR_LOCALEDIR is defined (for self contained bundles).