[Bf-codereview] Node linking operator supports multiple links at once (issue 6206070)

[lukas.toenne at googlemail.com]

Reviewers: bf-codereview_blender.org,

Description:
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

Please review this at http://codereview.appspot.com/6206070/

Affected files:
   source/blender/editors/space_node/node_draw.c
   source/blender/editors/space_node/node_edit.c
   source/blender/editors/space_node/node_intern.h
   source/blender/editors/space_node/node_ops.c