Fixes window bg randomly going crazy on startup

After recent auto-layout changes the default `auto` settings for main textured window's content border thickness goes crazy, as a result we get weird title bar light background:
![Zrzut ekranu 2013-01-30 o 00 29 58](https://f.cloud.github.com/assets/103067/109133/03e54f26-6a6c-11e2-93d7-9e6814ddb34d.png)

This patch provides simple workaround setting fixes zero border with `setContentBorderThickness:` as described at:
http://stackoverflow.com/questions/5812593/nswindow-textured-gradient-fill-weirdness
http://stackoverflow.com/questions/7795505/nswindow-textured-background-with-nstextfield/11482772#11482772