Code review comment for lp:~raof/do-plugins/plugins-i18n

Make the plugins use the translations we have. This (mostly) fixes bug #321748.

This essentially replaces all Mono.Unix.Catalog use with Mono.Addins.AddinManager.CurrentLocalizer. The .addin.xml then provides Mono.Addins with the necessary metadata to load the appropriate translations.

The addin metadata itself is not translated; Preferences->Plugins will still be in all-English. Fixing this last part will require one of two things
a) Mono.Addins to gain translation support for the addin metadata, a feature which is planned but not started, or
b) For us to change the plugin API and require translated properties on all plugins.

For the moment, I think we should merge this & release with it. We can look at the Preferences->Plugins list issue later.