systray [4/4]: implement systray transparency

[Kirill K. Smirnov]

> We don't want to export such a specific function. Besides, it should
> really be setting the window shape.
I've just understood why we cannot rely upon setting window shape to get 
transparency. Let's consider almost empty icon or even empty - this may 
happen when application "flashes" icon, e.g. on incoming ICQ message.
1) Can be there active windows with empty visible area?
2) This windows do not correctly relay mouse events.

--
Kirill