Make $translation_adapter public so it can be used by custom modules to add their own translations

Since Zend_Registry::get('Zend_Translate') is deprecated, a work around is to use parent::__construct() in the module constructor.

But if you make the WT_I18N variabele $translation_adapter public then this adapter can be used by custom modules to add their own translations and isn't it necessary to keep the deprecated function alive.

A custom translation then can be implemented in the module constructor in this way:
```
WT_I18N::$translation_adapter->addTranslation(
		new Zend_Translate('gettext', WT_MODULES_DIR.$this->getName().'/language/'.WT_LOCALE.'.mo', WT_LOCALE)
);
```