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

include/wx/html/helpfrm.h

@@ -115,9 +115,11 @@ public:
     // Override to add custom buttons to the toolbar
     virtual void AddToolbarButtons(wxToolBar* WXUNUSED(toolBar), int WXUNUSED(style)) {}
 
+    void SetShouldPreventAppExit(bool enable);
+
     // we don't want to prevent the app from closing just because a help window
     // remains opened
-    virtual bool ShouldPreventAppExit() const { return false; }
+    virtual bool ShouldPreventAppExit() const { return m_shouldPreventAppExit; }
 
 protected:
     void Init(wxHtmlHelpData* data = NULL);
@@ -143,6 +145,7 @@ protected:
     wxString m_TitleFormat;  // title of the help frame
     wxHtmlHelpWindow *m_HtmlHelpWin;
     wxHtmlHelpController* m_helpController;
+    bool m_shouldPreventAppExit;
 
 private: