Drag&drop moving of nodes in the node tree

[pschiffmann]

New nodes can only be inserted into empty regular slots, or in the last position of a collection slot. After creating a node, it can't be moved to a different place; if the node needs to be moved, the user has only two options: 1. Delete the node and create a new one at the target location with the same properties and descendants, or 2. manually edit the scene.json file.

The editor needs to support drag&drop gestures inside the node tree, allowing users to move nodes to a new location.