BUG: Ensure qm translation files are properly considered

The message "File ... qSlicerBaseQTCore_fr.qm" doesn't exist."
won't occur anymore when starting Slicer.

See http://www.na-mic.org/Bug/view.php?id=2863

Fixes #2863

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21574 3bd1e089-480b-0410-8dfb-8563597acbee