Window opacity option

```
I'd love to be able to lower the opacity of entire Xee windows and images when 
"floating on top" is enabled so that I can open reference images for art I'm 
making in Xee and still see photoshop, zbrush or whatever underneath.
```

Original issue reported on code.google.com by `shaunbar...@gmail.com` on 9 Feb 2015 at 1:05
