Add English translations

Automatically generate empty _en.qm files. This allows the user to
use the QTranslator::load(QLocale) overload, e.g.

  translator.load(QLocale(), "qt", "_", QLibraryInfo::location(QLibraryInfo::TranslationsPath));

So far this has been using a non-English translation even if English
has a higher priority (for instance by setting LANGUAGE=en:de on Linux).

[ChangeLog][Translations] Qt modules now include an English translation
(_en.qm) file. This fixes the behavior of QTranslator::load(QLocale) if
English is just one of the configured languages.

Change-Id: Ibb9e721ef501fbf5cb231671e3b89ef27a68f0a2
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>