Use a kernel event object to signal the thread wake up instead of sending WM_NULL to one of its windows. This is simpler as we don't need to look for any windows and doesn't suffer from the problem of overflowing the Windows message queue if we do it too many times as signalling an already signalled event simply doesn't do anything. Closes #9053. git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78041 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775