Make wxThread::OnExit() protected rather than private

This allows calling the base class version from the derived classes and
also, at least as importantly, make sure that the function is actually
documented as private functions are not extracted by Doxygen by default.

Closes #16749.

Closes https://github.com/wxWidgets/wxWidgets/pull/1543

include/wx/thread.h

@@ -622,16 +622,16 @@ protected:
     // in the context of the thread that called Kill().
     virtual void OnKill() {}
 
+    // called when the thread exits - in the context of this thread
+    //
+    // NB: this function will not be called if the thread is Kill()ed
+    virtual void OnExit() {}
+
 private:
     // no copy ctor/assignment operator
     wxThread(const wxThread&);
     wxThread& operator=(const wxThread&);
 
-    // called when the thread exits - in the context of this thread
-    //
-    // NB: this function will not be called if the thread is Kill()ed
-    virtual void OnExit() { }
-
     friend class wxThreadInternal;
     friend class wxThreadModule;