Preferences come to the front under more circumstances and OSes.

- If Shroud is visible (normal case), then the preferences panel appears in front of all windows, without disturbing the backdrop ordering.
- If Shroud is hidden, the backdrop comes to the front, with the preferences panel above it.
- If Shroud is on another space on Mac OS X 10.6, the backdrop comes to the front, with the preferences panel above it.
- If Shroud is on another desktop on OS X 10.8, the backdrop comes to the front, with the preferences panel above it, but then you get a weird fading effect and the panel goes to the back.  I have no idea why.  The Mac OS X 10.6 Spaces workaround is not responsible for this.

This should *not* be this hard.
Update documentation to reflect newer information about using Spaces.