Bug 166643 - gimp support for the XDG basedir spec

New configuration directory scheme, consistent across platforms, and
following standards.

UNIX platforms (except OSX): $XDG_CONFIG_HOME/GIMP/{GIMP_APP_VERSION}
Windows: %APPDATA%/GIMP/{GIMP_APP_VERSION}
OSX: NSApplicationSupportDirectory/GIMP/{GIMP_APP_VERSION}