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/helpctrl.h

@@ -48,6 +48,8 @@ public:
     wxHtmlHelpController(int style = wxHF_DEFAULT_STYLE, wxWindow* parentWindow = NULL);
     virtual ~wxHtmlHelpController();
 
+    void SetShouldPreventAppExit(bool enable);
+
     void SetTitleFormat(const wxString& format);
     void SetTempDir(const wxString& path) { m_helpData.SetTempDir(path); }
     bool AddBook(const wxString& book_url, bool show_wait_msg = false);
@@ -129,6 +131,8 @@ protected:
     wxHtmlHelpFrame*    m_helpFrame;
     wxHtmlHelpDialog*   m_helpDialog;
 
+    bool                m_shouldPreventAppExit;
+
     wxDECLARE_NO_COPY_CLASS(wxHtmlHelpController);
 };