[Bf-codereview] Node socket selection reimplemented (issue 5727064)

[lukas.toenne at googlemail.com]

Patch set 2 modifies the link_make operator instead of creating a
separate one, so it takes selected sockets into account. When
auto-connecting two nodes this operator now prefers selected sockets
over automatically choosing by name/type if available. This means it can
freely combine nodes with and without selected sockets.

If any inputs are selected they will all be connected to the same output
in the previous node (snode_autoconnect builds a node chain from node
locations). The selected output will also be chosen either automatically
or from selection. Only one selected output per node is allowed, but
multiple outputs from different nodes are possible, to enable the chain
linking in snode_autoconnect.

NOTE: currently make_link operator does not support sockets in group
node interface lists. This would require further changes to
snode_autoconnect, but i hope it won't be needed due to changes in the
pending group nodes overhaul. That would replace group interface sockets
by regular nodes, so everything can be handled consistently.

http://codereview.appspot.com/5727064/