Remove xib settings that seem to crash InterfaceBuilder in 10.8

I have no idea how necessary the `appearanceType=“aqua”` properties on some of the IB objects are, but any instance of them seem to set off a 10.8 bug (even on XC 5.1.1) that will crash InterfaceBuilder and also `ibtools` when compiling. I was able to compile only after removing them. If you want MacDown to be build able on 10.8 this seems necessary.

See:
- <http://stackoverflow.com/questions/20531240/xcode-5-ibtool-failed-with-exit-code-255>
- <http://www.tyreeapps.com/blog/tools/2014/06/10/Xcode-Crash-with-xib-Files.html>