Fix setting translation domain on the Grantlee localizer

Summary:
GrantleeTheme::Engine::localizer is a factory function, so calling
methods on a temporary returned by it has no effect.

Reviewers: aacid, mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D27443