Blender Git Loki

Cleanup: Simplify node group input and output socket verification

This commit refactors the way the socket lists for group nodes,
and group input/output nodes are verified to match the group's
interface.

Previously the `bNodeSocket.new_sock` pointer was used to
temporarily mark the new sockets. This made the code confusing
and more complicated than necessary.

Now the old socket list is saved, and sockets are moved directly from
the old list to a new list if they match, or a new socket is created
directly in the new list.

This change is split from D13518, which aims to remove the `new_node`
and `new_sock` pointers. In the future this code might be removed
entirely in favor of using node socket declarations.

Differential Revision: https://developer.blender.org/D13543