Dialog: force the dialog to be visible before trying to position it s…

…o we can get the current offset (required for using .offset() as a setter).

ui/jquery.ui.dialog.js

@@ -421,6 +421,7 @@ $.widget("ui.dialog", {
 			}
 		}
 
+		// need to show the dialog to get the actual offset in the position plugin
 		var isVisible = this.uiDialog.is(':visible');
 		if (!isVisible) {
 			this.uiDialog.show();