Remove X11 dependency from OSX.

This removes Windows.Forms from the game and utility, which combine with the custom libSDL2.dylib in #5190 to remove our dependency on X11/XQuartz under OSX.

This fixes the excessive font-caching delay (#4785) and looks much nicer:

![dialogs](https://cloud.githubusercontent.com/assets/167819/2871519/de7d8d42-d311-11e3-94d2-3e2dbfc87932.png)