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/infobar.h

@@ -36,7 +36,8 @@ public:
     virtual void ShowMessage(const wxString& msg,
                              int flags = wxICON_INFORMATION) = 0;
 
-    // add an extra button to the bar, near the message
+    // add an extra button to the bar, near the message (replacing the default
+    // close button which is only shown if no extra buttons are used)
     virtual void AddButton(wxWindowID btnid,
                            const wxString& label = wxString()) = 0;