Add new api for appending a separator to the thumbnail toolbar.

Actually, when calling AppendSeparator we are appending a disable
ThumbBarButton without background, which can simulate the behavior
of appending a separator.

Author: Chaobin Zhang

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77610 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/taskbarbutton/taskbarbutton.cpp

@@ -346,13 +346,12 @@ void MyFrame::OnSetOrRestoreThumbnailClip(wxCommandEvent& event)
     MSWGetTaskBarButton()->SetThumbnailClip(rect);
 }
 
-
 void MyFrame::OnAddThubmBarButton(wxCommandEvent& WXUNUSED(event))
 {
     if ( m_thumbBarButtons.size() >= 7 )
         return;
 
-    wxThumbBarButton* button =
+    wxThumbBarButton *button =
         new wxThumbBarButton(m_thumbBarButtons.size() + ThumbnailToolbarBtn_0 ,
                              CreateRandomIcon());
     MSWGetTaskBarButton()->AppendThumbBarButton(button);