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

docs/doxygen/overviews/app.h

@@ -27,8 +27,7 @@ the wxApp::OnInit member defined for a class derived from wxApp.
 @e OnInit will usually create a top window as a bare minimum. Unlike in earlier
 versions of wxWidgets, OnInit does not return a frame. Instead it returns a
 boolean value which indicates whether processing should continue (@true) or not
-(@false). You call wxApp::SetTopWindow to let wxWidgets know about the top
-window.
+(@false).
 
 Note that the program's command line arguments, represented by @e argc and
 @e argv, are available from within wxApp member functions.
@@ -59,7 +58,6 @@ bool DerivedApp::OnInit()
     wxFrame *the_frame = new wxFrame(NULL, ID_MYFRAME, argv[0]);
     ...
     the_frame->Show(true);
-    SetTopWindow(the_frame);
 
     return true;
 }