Bug 655

I would like to propose a change to Popup.php in which all the popup windows 
are centered on the user's screen. This would involve a little bit of 
javascript and a change to the window.open function to include the top and left 
parameters. Question is where the general parameters (in my case w and h) 
should be stored, as we don't want to repeat them for every popup.

var w = 480, h = 340;
if (document.all || document.layers) 
{
   w = screen.availWidth;
   h = screen.availHeight;
}
		
var popW = 990, popH = 655;
var leftPos = (w-popW)/2, topPos = (h-popH)/2;