Split translations for plug-ins

[Lucian Adrian Grijincu <lucian grijincu gmail com>]

As rygel was added to GNOME 3.0 people will begin contributing more
translations to this project.

Currently there's a single file with all the translations.

This is manageble at this point as there are only 194 translations,
but, as more plug-ins are added. This will become a very large file.

Worse is the fact that some plug-ins will not be interesting to the
general public.
For example rygel-mediathek. It's only for a german TV channel. Not
many non-german speakers will be interested in this plug-in. Because
of this translations for this module are of a lower priority.

Because of the current jumbo-file setup, it's harder to differentiate
between strings for a plug-in, for another plug-in or for main rygel,

Doing this work later will impose a greater work for the translators
who translate rygel before such a change, as they will need to port
translations from one file to another.

-- 
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..: Lucian