Node socket selection reimplemented

In 2.49 you could select not only nodes but also individual sockets. This is a helpful feature when it comes to creating links over long distances in the editor. The current click&hold + scrolling is very cumbersome, and the auto-connect feature between nodes is limited and not easily controllable.

This patch reimplements the socket selection by adding two new operators, which toggle the SELECT flag on sockets based on the already used lookup function from linking. Default mapping is SELECTMOUSE (rmb).

Selection state is shown as a simple white outline on sockets. This can be tweaked easily according to user feedback once in trunk.
http://www.pasteall.org/pic/27820

Socket selection is limited to a single socket on the output side, since any input can only be linked to one output. In the future there may be special socket types that deviate from this "one to many" behavior. In that case all the operators will have to be checked anyway, most of them rely on this behavior to some extent.

In addition to the new selection operators the "make link" operator has also changed:

* The existing NODE_OT_link_make now uses socket selection for making links (default key mapping: FKEY, CTRL+FKEY for replacing existing links) (old link_make operator even still had all the 2.49 comments saying it uses socket selection ...)
* A new NODE_OT_link_auto has been added with the old autoconnect behavior (key map ALT+FKEY, ALT+CTRL+FKEY)

Not sure if this is a good change, but IMHO linking by socket selection is much more useful than autoconnect.

[brechtvl, 2012-03-06 13:39:29]

There does not seem to be any obvious way to deselect sockets, so when making
links I'm now always left with unwanted selected sockets. Clicking a second time
on the socket, or dragging to make a link should deselect the socket I think.

Maybe this tool could be simpler, and not rely on a make links operator at all:
just click once on a socket to make it selected, and then click on another
socket to make a link it. That way the existing operator can keep working, and
it's faster to use too.

[lukas.toenne, 2012-03-07 10:19:26]

Same operator used for socket and node connect

[lukas.toenne, 2012-03-07 10:28:30]

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.

[lukas.toenne, 2012-03-08 16:46:00]

Avoid deselecting all sockets when selecting nodes a second time. Deselect all
sockets after linking

[brechtvl, 2012-03-08 20:08:26]

Now I can't select sockets at all? Probably because it gets unselected after
dragging. If you fix that, LGTM to commit.

[lukas.toenne, 2012-03-09 10:06:01]

Ok, this is a bit tricky due to keymap inconsistency (SELECTMOUSE/ACTIONMOUSE
for select, LEFTMOUSE for link) and the need for operator pass-through, but i
think i have a nice solution. Problem is that in order to get to the link
operator the selection always has to return pass-through, but if socket select
does that then the node select op will deselect all previous nodes and sockets
again ...

What i did now is merge the socket selection operators into the existing node
select operator (sockets checked first, then nodes). This way the socket/node
blocking from overlap can be done completely internal in the operator, which is
also nicer because sockets are part of nodes after all.

Also sockets only get deselected when the link operator is successful (not on
cancel), so left click can still be used for socket select too.

The autoconnect operator could still use some tweaking to make it more reliable,
but i'd rather do that in a separate patch later. Tests look good, so will
commit this to trunk today.