PinJoint2D unpins when parent node moved

[mcardellg]

Godot version:
3.1.1

OS/device including version:
Windows 10

Issue description:
When moving a node using remove_child + add_child any pin joints that are children of the node get unpinned. The joint copies correctly but detaches from it`s pinned nodes.

Steps to reproduce:
Run the attached minimal project, press enter to move the node in the tree, note that the previously pinned sub-node becomes detached but that the tree looks as expected.

Minimal reproduction project:
unpinbug.zip

[KoBeWi]

Still valid in 6f4384f