QTranslator: don't repeatedly re-create a QString from a QLatin1String

... each time around the loops. Cache it.

Also use QStringLiteral, since the string will never be
modified.

Also saves 96b in text size on optimized GCC 4.9 Linux
AMD64 builds.

Change-Id: I0269586235da18f3073a553739561ea7db6356e8
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>