Add new pohelper plugin

Translation Helper is a plugin for Geany that improves the support for GetText translation files, by providing various features specific to this format and to translators.

Features
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* Navigation between all, untranslated or fuzzy messages;
* Reformatting of the translation (reflow);
* Toggling the fuzziness of a translation;
* Pasting of the untranslated string to the translation;
* Automatic updating of the translation metadata.