//funcio que obre un popup centrat a la pantalla
function OpenWindowCentered(theURL, width, height, featuresPrefix) {
	var aw = screen.availWidth - 10;
	var xc = (aw - width) / 2;
	features = "menubar,status,scrollbars,resizable, top=0, left=" + xc + ",screenX=" + xc+", width=" + width + ", height=" + height;
	if (featuresPrefix!=null)
	  features=featuresPrefix+features;
	window.open(theURL,'',features);
	
}