Make main thread wake up code more efficient and less error-prone in wxMSW.

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

include/wx/msw/evtloop.h

@@ -53,7 +53,6 @@ public:
     // override/implement base class virtuals
     virtual bool Dispatch();
     virtual int DispatchTimeout(unsigned long timeout);
-    virtual void WakeUp();
 
 protected:
     virtual void OnNextIteration();