Flexible node groups in cycles

This allows group nodes inside other group nodes in cycles and makes the code more generic for all possible cases, like direct group input-to-output links and unused group sockets.

Previous code tried to connect external nodes and internal group sockets by following links until a "real" node input/output. This quickly becomes complicated in corner cases as described above and can lead to unexpected behavior when the group socket is of a different type than the internal/external sockets, but that conversion is skipped.

The new code uses the concept of "proxy nodes" similar to what the new compositor does. Each group socket is replaced with a proxy node with a single input and output, to which other nodes in the same tree and internal nodes can link to. After all groups have been expanded in the graph, these proxy nodes are removed again, adding converter nodes if necessary.

[brechtvl, 2011-12-16 16:25:21]

Can find nothing to comment on, LGTM :)