Add wxInfoBar::RemoveButton() method.

Also change the GTK implementation to use a separate wxInfoBarGTKImpl to store
its data, this object won't be even allocated if a generic implementation is
used under GTK.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62277 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/infobar.h

@@ -131,6 +131,16 @@ public:
      */
     void AddButton(wxWindowID btnid, const wxString& label = wxString());
 
+    /**
+        Remove a button previously added by AddButton().
+
+        @param btnid
+            Id of the button to remove. If more than one button with the same
+            id is used in the info bar (which is in any case not recommended),
+            the last, i.e. most recently added, button with this id is removed.
+     */
+    void RemoveButton(wxWindowID btnid);
+
     /**
         Show a message in the bar.