Build language list from Subsurface translations

On Windows platform translation/ folder contains Qt translation
files as well. This results in extra languages in preferences.
From now we filter these translations and choose only those matching
^subsurface_*.qm$

Fixes #339

Signed-off-by: Sergey Starosek <sergey.starosek@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>