Replace plugin_lang_get with new gettext approach

The replacement of lang_get() calls for strings in the 'core' text
domain has mostly been completed. Plugins were calling a separate
function plugin_lang_get() that told lang_api (now deprecated) to use
the plugin-specific "lang" directory for translations specific to
plugins.

This new approach loads a text domain specific to each plugin prior to
plugin initialisation. Just as MantisBT core has a "locale" directory,
each plugin also needs a "locale" directory.

plugin_lang_get(...) has been completely removed and d___(...) and
dn_(...) should be used instead. This new syntax may change to something
simler and easier to handle in the near future.

Commit: d956d32