Add wxHtmlHelpController::SetShouldPreventAppExit().

Add a method which can be used to indicate that the help window should prevent
the application from exiting and use it in the help sample to prevent it from
closing prematurely.

Closes #13046.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67228 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/html/helpview/helpview.cpp

@@ -86,6 +86,8 @@ bool MyApp::OnInit()
     delete wxLog::SetActiveTarget(new wxLogGui);
 #endif
 
+    help->SetShouldPreventAppExit(true);
+
     help -> DisplayContents();
 
     return true;