Add support for wxHELP button to wxMessageDialog.

Implement support for wxHELP for wxMSW, wxGTK and wxOSX/Cocoa (at least when
showing the message box from the main thread, there doesn't seem to be any way
to show more than three buttons with CFUserNotificationDisplayAlert() so
"Help" button is not supported when using it).

This is useful not only on its own, i.e. to allow the user to ask for help,
but also because it brings the total number of buttons supported by the
message dialog to 4, meaning that more choices can be offered to the user
(which is rarely, but not quite never, useful).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68537 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msgdlg.h

@@ -177,10 +177,16 @@ public:
         return true;
     }
 
+    virtual bool SetHelpLabel(const ButtonLabel& help)
+    {
+        DoSetCustomLabel(m_help, help);
+        return true;
+    }
+
     // test if any custom labels were set
     bool HasCustomLabels() const
     {
-        return !(m_ok.empty() && m_cancel.empty() &&
+        return !(m_ok.empty() && m_cancel.empty() && m_help.empty() &&
                  m_yes.empty() && m_no.empty());
     }
 
@@ -195,6 +201,8 @@ public:
         { return m_ok.empty() ? GetDefaultOKLabel() : m_ok; }
     wxString GetCancelLabel() const
         { return m_cancel.empty() ? GetDefaultCancelLabel() : m_cancel; }
+    wxString GetHelpLabel() const
+        { return m_help.empty() ? GetDefaultHelpLabel() : m_help; }
 
     // based on message dialog style, returns exactly one of: wxICON_NONE,
     // wxICON_ERROR, wxICON_WARNING, wxICON_QUESTION, wxICON_INFORMATION
@@ -250,6 +258,7 @@ protected:
     const wxString& GetCustomYesLabel() const { return m_yes; }
     const wxString& GetCustomNoLabel() const { return m_no; }
     const wxString& GetCustomOKLabel() const { return m_ok; }
+    const wxString& GetCustomHelpLabel() const { return m_help; }
     const wxString& GetCustomCancelLabel() const { return m_cancel; }
 
 private:
@@ -259,13 +268,15 @@ private:
     virtual wxString GetDefaultNoLabel() const { return wxGetTranslation("No"); }
     virtual wxString GetDefaultOKLabel() const { return wxGetTranslation("OK"); }
     virtual wxString GetDefaultCancelLabel() const { return wxGetTranslation("Cancel"); }
+    virtual wxString GetDefaultHelpLabel() const { return wxGetTranslation("Help"); }
 
     // labels for the buttons, initially empty meaning that the defaults should
     // be used, use GetYes/No/OK/CancelLabel() to access them
     wxString m_yes,
              m_no,
              m_ok,
-             m_cancel;
+             m_cancel,
+             m_help;
 
     wxDECLARE_NO_COPY_CLASS(wxMessageDialogBase);
 };