enable way to trigger reorder after move

[ElectricMaxxx]

That are the changes to finally get the dnd working.

Todos:

• move
• sort on tree

[ElectricMaxxx]

I definded options on sonata we need here now. But what we still miss is the position value in the resource, and the ability of the tree to order by it.

[ElectricMaxxx]

sortChildren() sorts the tree on the first request in the right way as i have the json data (but i do not see the call at all) when i want to sort again i have fancytree nodes and the method does not work.

[ElectricMaxxx]

@symfony-cmf/contributors Works on Sandbox, pls review. Working example:
symfony-cmf/cmf-sandbox#366