use kind, not id, of a menu item to test whether it's a separator: this allows having...

use kind, not id, of a menu item to test whether it's a separator: this allows having separators with ids other than wxID_SEPARATOR in case it's useful to distinguish between them (patch 1929039)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@53056 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775