Improved i18n for plugin

(1) revised translation loading call in cfs.php:
- avoid hard-coded plugin slug in load_plugin_textdomain()
- additional location for translation files in WP_LANG_DIR subfolder (won't be overridden on plugin updates)
- if lang files in both locations they will be merged, with priority for WP_LANG_DIR ---> see WordPress core in wp-includes/l10n.php for principle
- usage: this will enable users to place their (custom) translations in wp-content/languages/custom-fuild-suite/ which are then update-safe :-)

(2) unified all textdomains to 'cfs' alone

(3) added some of missing i18n/Gettext syntax (quite a few strings... :)

(4) updated default .po/.pot file for translators (.po file is better handled in a lot of editors...)

(5) completed, corrected and improved packaged German translations (now proper formal version; in the light of above changes & additions)

Thanks, Dave :)