void tree_DragDrop(object sender, Gui.Controls.DragEventArgs e)
 {
     if (((IDataObject)e.Data).GetDataPresent(DataFormats.FileDrop))
     {
         var filename = (string)((IDataObject)e.Data).GetData(DataFormats.FileDrop);
         OnFileDropped(new FileDropEventArgs(filename));
     }
 }
 void tree_DragOver(object sender, Gui.Controls.DragEventArgs e)
 {
     if (((IDataObject)e.Data).GetDataPresent(DataFormats.FileDrop))
     {
         e.Effect = e.AllowedEffect & Gui.Controls.DragDropEffects.Copy;
     }
     else
     {
         e.Effect = Gui.Controls.DragDropEffects.None;
     }
 }