Fix Issue #9: Drag ListBoxItem that includes a TextBox. 
Fix Issue #9: Drag ListBoxItem that includes a TextBox. 

When a TextBox is included in a list box's data template, System.Windows.DragDrop.DoDragDrop() ends up being called twice, which results in m_DragInfo being null when the second call to the method exits. Make sure that DoDragDrop is only called once.

git-svn-id: 0c1fea0d041f2968ed8210d06e1598c17ac6d68f@22 10a697b4-cb8d-11de-a774-41d3a1f294e1

emsaks/gong-wpf-dragdrop,Livit/Mailbird.Gong.WPF.DragDrop,punker76/gong-wpf-dragdrop,Haukinger/gong-wpf-dragdrop,ruisebastiao/gong-wpf-dragdrop,huoxudong125/gong-wpf-dragdrop,JohnnyJoe/Johnny-s-repository,JohnnyJoe/Johnny-s-repository