support listing translation files as sources

source strings from such files are marked as obsolete; the only purpose
is collecting possible translations.

this is an obscure feature meant to facilitate the modularization of the
qt catalogs. it's hard to describe in a few words (the format needed for
-help), so leaving it undocumented (for now).

Change-Id: I744ec7533425aee741b909bccf51442ca5241b10
Reviewed-by: hjk <qthjk@ovi.com>