Move plugin_lang_defaulted() function to MantisBT core

Created by: dregad

#215 introduced a new plugin_lang_defaulted() function in Source.API.php.

As mentioned in https://github.com/mantisbt-plugins/source-integration/pull/215#issuecomment-308025327 this function really belongs in MantisBT core / plugin_api.php so it should be moved there and eventually removed from the plugin's API.