Qt Designer: Load translations only when designer_<LANG>.qm is present.

Try to load designer_<LANG>.qm first and load qt_<LANG>.qm only
if the former succeeds.

Task-number: QTBUG-41790
Change-Id: Id4862019edf81da81bf9d059c586bbfee18ab7e6
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>