Add a "not specified" icon style to the message box test dialog.

The sample allowed to test the behaviour with all wxICON_XXX including
wxICON_NONE but not without any icon styles at all. Do allow it now.

Also use symbolic names for the radiobox indices for clarity.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63681 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/dialogs/dialogs.h

@@ -214,6 +214,17 @@ private:
 
     static const BtnInfo ms_btnInfo[Btn_Max];
 
+    enum
+    {
+        MsgDlgIcon_No,
+        MsgDlgIcon_None,
+        MsgDlgIcon_Info,
+        MsgDlgIcon_Question,
+        MsgDlgIcon_Warning,
+        MsgDlgIcon_Error,
+        MsgDlgIcon_Max
+    };
+
     wxTextCtrl *m_textMsg,
                *m_textExtMsg;