#1214 ACP language editor improved

* finds possible translation problems, missing or extra files and show hints on list of files in editwith mode
* finds possible translation problems and show hints while editing in editwith mode
* allow deletion of obsolete extra phrases from old language packs (this option is not available when editing original mybb language pack or config_settings.lang.php)
* increased stability
* allows use of copy paste to aid translation
* fixes bug where part of variables to which we compare are not displayed if not exists in edited file while in editwith mode