Fix apparent activation loss after hiding previous popup

Commit 58d4b0e209 introduced a regression:
if a previous popup still existed when the new one was shown, dismissing
the second popup would result in the owner window losing its "active"
appearance.

This was due to the fact that ::GetActiveWindow() still returned the
former popup when it was about to be dismissed, so it was too early to
call it in WM_NCACTIVATE handler. Do it now in DismissOnDeactivate()
itself which is both simpler and more correct.

include/wx/popupwin.h

@@ -152,7 +152,7 @@ public:
                                   WXLPARAM lParam) wxOVERRIDE;
 
 private:
-    void DismissOnDeactivate(WXHWND hwndActive);
+    void DismissOnDeactivate();
 
     wxDECLARE_DYNAMIC_CLASS(wxPopupTransientWindow);
     wxDECLARE_NO_COPY_CLASS(wxPopupTransientWindow);