title: extjs4 - filter grid drag and drop
body: <p>I've got 2 grids with drag and drop enabled. </p>
  
  <p>On one of the grids I would need to check the dropped elements and filter out some of them.</p>
  
  <p>I can't quite figure out how to do this. The grid dd plugin has 2 events: beforedrop and drop.</p>
  
  <p>During the beforedrop event, I can check if each of the dropped records are ok, but there's no obvious way to select a subset of the records being dropped.
  The only option I seem to have is to return false if I want to cancel the drop entirely - whereas I want to drop only a select subset of records.</p>
  
  <p>The drop event seems too late.</p>
  
  <p>Any idea how I can do this?</p>