fix bug showing the CPComboBox's list under popover

AppKit/_CPPopUpList.j

@@ -140,6 +140,7 @@ var ListColumnIdentifier = @"1";
     [panel setTitle:@""];
     [panel setFloatingPanel:YES];
     [panel setBecomesKeyOnlyIfNeeded:YES];
+    [panel setLevel:CPStatusWindowLevel];
     [panel setHasShadow:YES];
     [panel setShadowStyle:CPMenuWindowShadowStyle];
     [panel setDelegate:self];