Fix resource localization

[LorenzCK]

At the moment Vernacular creates full platform resource files, which include untranslated (and untranslatable) entries in the POT/PO files.

The following entry, for instance

# Main application title
msgid "title_main"
msgstr ""

can be left untranslated in specific language files, but must be prevented to generate a resource entry in the platform files.

As of now, the values-it/strings.xml Android resource will contain the entry

<string formatted="false" name="Vernacular_P0_title_main">title_main</string>

which prevents the OS from localizing the string correctly from the default language resource files. (Same goes for the other platforms.)