Added ctypes Windows privilege detection

Thanks to [this](http://stackoverflow.com/questions/1026431/crossplatform-way-to-check-admin-rights-in-python-script/1026626#1026626) answer utilizing the ctypes module, the GUI now detects if the user has administrator privileges on Windows.

There are other approaches aswell mentioned on [Stack Overflow](http://stackoverflow.com/questions/2946746/python-checking-if-a-user-has-administrator-privileges).