Only show the default close button in wxInfoBar if there are no others.

Assume that user-added buttons can be already used to close the message so
don't show the default close button if any were added.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62280 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/infobar.h

@@ -57,6 +57,10 @@ protected:
 private:
     void Init() { m_impl = NULL; }
 
+    // add a button with the given id/label and return its widget
+    GtkWidget *GTKAddButton(wxWindowID btnid,
+                            const wxString& label = wxString());
+
 
     // only used when the native implementation is really being used
     class wxInfoBarGTKImpl *m_impl;