Node linking operator supports multiple links at once

A generalization of the modal node linking operator (for dragging from socket to socket).

This operator still had some built-in assumptions about the connectivity of input/output sockets (1-to-n in all current node systems). For future node systems (e.g. flow-based particles) and for general customizable nodes the operator is now fully symmetric and supports all kinds of connectivity limits (1:1, 1:n, m:1, m:n).

The operator data can also store a list of node links as opposed to a single link now, so that multiple links can be redirected at once. Holding the CTRL key when clicking a socket, all links from/to that socket are detached and can be moved to a different socket. This is useful for quickly appending a node without moving every individual link.

Here's a short demo:
http://www.pasteall.org/pic/31849

[brechtvl, 2012-05-15 12:34:49]

LGTM.

http://codereview.appspot.com/6206070/diff/1/source/blender/editors/space_nod...
File source/blender/editors/space_node/node_edit.c (right):

http://codereview.appspot.com/6206070/diff/1/source/blender/editors/space_nod...
source/blender/editors/space_node/node_edit.c:2479: /* XXX links->data pointers
are either held by the tree or freed already */
Does this "XXX" indicate that something needs to be fixed here? Otherwise I'd
leave it out.

[lukas.toenne, 2012-05-15 12:38:38]

http://codereview.appspot.com/6206070/diff/1/source/blender/editors/space_nod...
File source/blender/editors/space_node/node_edit.c (right):

http://codereview.appspot.com/6206070/diff/1/source/blender/editors/space_nod...
source/blender/editors/space_node/node_edit.c:2479: /* XXX links->data pointers
are either held by the tree or freed already */
On 2012/05/15 12:34:49, brechtvl wrote:
> Does this "XXX" indicate that something needs to be fixed here? Otherwise I'd
> leave it out.

Done.