Opacity when no maximized window

I added new option for opacity "Only when no maximized window":

![opacity-when-no-maximized](https://f.cloud.github.com/assets/4481613/630285/37239134-d19d-11e2-8bc3-6522dbcc03b5.png)

I like transparent background, but it's little disturbing when I have the dash visible, expanded over all height and some window is maximized.