primary screen WM property?

[Christopher Blizzard <blizzard redhat com>]

Both the Mac and Windows have the ability to label a screen as the 
primary screen.  This would be the window where you would find the 
toolbar, the window that new windows usually come up on, etc.  Xinerama 
doesn't have an API that describes a specific screen as the primary 
screen, and the newer Xinerama APIs on sourceforge don't appear to, either.

It would be nice if there was a way to describe one of the screens with 
Xinerama as the primary one so that toolbars could be properly placed 
and new windows would come up on that primary window.

In theory, we could just create a property on the display that said 
which one of the screens is the primary.  The WM spec seems like a good 
place to do this since the WM is responsible for window placement. 
Sound reasonable?  The property description should probably be dead simple.

What do folks think?

--Chris