[Trinidad] js centering bug in tr:panelPopup

[Renzo Tomaselli]

Hi, in method TrPanelPopup._centerOnScreen() the window height is 
computed for IE through document.body.clientHeight.
This provides document height instead of window height, in case of IE 6+ 
in strict mode.
The final result is an up-shifted panel, even beyond the top in case of 
panels higher than current document.
Using document.documentElement.clientHeight does the right job, if 
returned value is not 0 (strict). Otherwise we are in quirks mode, thus 
use document.body.clientHeight.