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/generic/infobar.h

@@ -43,6 +43,7 @@ public:
 
     virtual void AddButton(wxWindowID btnid, const wxString& label = wxString());
 
+    virtual void RemoveButton(wxWindowID btnid);
 
     // methods specific to this version
     // --------------------------------