rewrite support for listing translation files as sources

ts files listed among the sources now contribute extant source strings
with locations, i.e., they are treated like all other sources. this
basically makes it possible to use lupdate like gettext's msgmerge.
this enables us to write a much more efficient variant of the
split-qt-ts.pl script found in qttranslations.

the original functionality of adding ts files as providers of
additional translations to merge is hidden behind the new -xts flag; it
is of course still undocumented.

Change-Id: I7a5488d814f39d6b5e9053a414547a6b1461fc14
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>