Store settings file in "proper" location.
Store settings file in "proper" location.

I'm tired of the settings file polluting the home directory. The program
now tries to intelligently pick a directory to store the file, based on
the current operating system:

  Mac OS X:  ~/Library/Preferences/3ddv.xml
  Linux/BSD: ~/.3ddv.xml
  Others:    Default to $HOME/3ddv.xml (same as before)

mdippery/3ddv,mdippery/3ddv