Dispatch events from the native MSW wxProgressDialog too

Do this for compatibility with wxGenericProgressDialog, which always did
this and can't really do otherwise as it needs to react to the clicks on
its buttons, and also because not doing it results in the other
application windows being marked as "not responding" by MSW, which looks
bad.

Closes #17768.

include/wx/msw/progdlg.h

@@ -59,6 +59,10 @@ private:
     // Returns false if the user requested cancelling the dialog.
     bool DoNativeBeforeUpdate(bool *skip);
 
+    // Dispatch the pending events to let the windows to update, just as the
+    // generic version does. This is done as part of DoNativeBeforeUpdate().
+    void DispatchEvents();
+
     // Updates the various timing informations for both determinate
     // and indeterminate modes. Requires the shared object to have
     // been entered.