Use the raster rendering engine by default on OSX

Seems like using the raster engine fixes performance issues on OSX,
and there seem to be no noticeable side-effects compared to native
rendering. So let's use this by default.

If you notice issues, please let us know. Also, you can always enforce
native rendering via command line:

open Quassel\ Client.app -W --args -graphicssystem native

Thanks to rikai for figuring this out!
Update translations from Transifex

  25507 translated messages

Many thanks to:
 - fi: Larso <larso@gmx.com>
 - tr: volkangezer <volkangezer@gmail.com>
 - uk: Yuri Chornoivan <yurchor@ukr.net>, Daniel Albers <daniel@lbe.rs>
 - pt_BR: André Marcelo Alvarenga <alvarenga@kde.org>
Update quassel.pot
These two settings keys shouldn't be translated
Merge https://github.com/lliehu/quassel into i18n-tx-sync
Update quassel.pot