FlexTreeGrid withDragDrop for XML

I played a bit with the flexlib TreeGrid from Yaniv De Ridder which is really great! Its has still some bugs in flexlib version 2.2 and some of the features of the extended DataGrid do not work anymore as sorting columns and dragdrop, but I believe they will in a future version.

Until then I wanted some features to work in the TreeGrid which are

1. Treegrid should update when dataprovider changes (there is a workaround for that : issue 15), and
   
1. openItems should stay open when a child is added or removed from the dataProvider
2. scrollPositions should stay when a child is added or removed from the dataProvider
2. drag and drop of elements from one tree item to another
   
1. it should be possible to ativate drag drop only work if the items will be placed at the same depth

My approaches are just a first try and maybe there are still some bugs. So I would be glad to receive comments and improvements from you.