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

interface/wx/html/helpctrl.h

@@ -171,6 +171,21 @@ public:
     virtual void ReadCustomization(wxConfigBase* cfg,
                                    const wxString& path = wxEmptyString);
 
+    /**
+        Sets whether the help frame should prevent application from exiting
+        if it's the only remaining top level window.
+
+        @enable
+            If @true, the application will not quit unless the help frame is
+            closed. Default is @false, i.e. the application does exit if only
+            the help window remains opened.
+
+        @see wxApp::SetExitOnFrameDelete()
+
+        @since 2.9.2
+    */
+    void SetShouldPreventAppExit(bool enable);
+
     /**
         Sets the path for storing temporary files - cached binary versions of index and
         contents files.