Prevent WM_CONTEXTMENU from being propagated upwards the window parent chain
by DefWindowProc(), we already do it ourselves and not marking the message as
processed could result in multiple calls to the same wxEVT_CONTEXT_MENU
handler if it skipped the event.
See #13683.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73949 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775