Provide a package text domain loader

[jlambe]

Description

In order to smooth package development and provide l10n and i18n support using gettext methods, it would be best that the framework defines a common function in order to load .mo files.

Methods like load_plugin_textdomain do not work as they require the package to be installed as a plugin. So instead, we should use the general function load_textdomain and provide a generic package structure where to store the translation files. For example inside a package resources/languages directory.

Now I'm not sure about best practice for the filename. Two options come to mind:

1. Compose the file name using the "domain name" and "locale" like so: textdomain-en_US.mo
2. Organize by subdirectory using the "locale" and each directory contains one file based on domain: en_US/textdomain.mo

The first option will contain all files into one languagesdirectory while the second options will contains many subdirectories based on locale with one file into each.

First option

+-- languages/
|    +-- textdomain-en_US.mo
|    +-- textdomain-en_US.po
|    +-- textdomain-fr_FR.mo
|    +-- textdomain-fr_FR.po

Second option

+-- languages/
|    +-- en_US/
     |    +-- textdomain.mo
     |    +-- textdomain.po
|    +-- fr_FR/
     |    +--textdomain.mo
     |    +--textdomain.po