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!