Remove calls to wxApp::SetTopWindow() from the samples and documentation.

It is definitely not necessary to call SetTopWindow() when there is only a
single top level window and it is arguable whether it's useful to do it even
when there are many of them so don't encourage its use in the documentation
and also remove all its occurrences from the samples.

Closes #12816.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66528 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/exec/exec.cpp

@@ -413,9 +413,8 @@ bool MyApp::OnInit()
     MyFrame *frame = new MyFrame(wxT("Exec wxWidgets sample"),
                                  wxDefaultPosition, wxSize(500, 140));
 
-    // Show it and tell the application that it's our main window
+    // Show it
     frame->Show(true);
-    SetTopWindow(frame);
 
     // success: wxApp::OnRun() will be called which will enter the main message
     // loop and the application will run. If we returned false here, the