process one event at once in wxEvtHandler::ProcessPendingEvents() to prevent crashes...

process one event at once in wxEvtHandler::ProcessPendingEvents() to prevent crashes when a (pending) event handler destroys the event handler object itself; only add the event handler to wxPendingEvents list if it's not already there (and explicitly mention that an object can be present in this list only once in the comment) (replaces patch 1837719)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@51021 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775