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

include/wx/infobar.h

@@ -40,6 +40,9 @@ public:
     virtual void AddButton(wxWindowID btnid,
                            const wxString& label = wxString()) = 0;
 
+    // remove a button previously added by AddButton()
+    virtual void RemoveButton(wxWindowID btnid) = 0;
+
 private:
     wxDECLARE_NO_COPY_CLASS(wxInfoBarBase);
 };