Blender Git Loki

UI: Draw real node sockets for node-input buttons and left-align them

Couple of reasons for this:
* Makes it easier to understand that and how these buttons map to nodes.
* These buttons represent node inputs, so it makes more sense to have the icons
on the left.
* We'd like to show the usual decorators on the right, but having two circles
for different purposes next to each other would be confusing.

Also respects the socket shape. So if a diamond shape is used for example, the
properties show that too.

I'm not sure if the previous icon textures were cached, either way this method
doesn't use the caching. So this adds a draw call for every socket icon which
is a bit annoying, but probably neglectable. Would be better if we'd use proper
icon textures or batches, but that's a separate change.

Differential Revision: https://developer.blender.org/D7409