Blender Git Loki

Fix #33628, Segmentation fault after pasting a closed group of nodes into an open group. Finally now there is a proper check for pasting nodes into groups. It uses the poll_instance callback of node types to determine if a node can be added into a specific node tree. Currently this is only implemented for group nodes and does a recursive check to avoid pasting a node group into itself (on any level, also nested groups).