Add a get_location () Function to Node Sockets

Currently, there is no easy way to access the location of a node socket in the node graph. This makes it very difficult to do custom link drawing operations. If a get_location function was added, it would return a 2D vector for the location of the socket (not location on the node, but location in the node editor space). On a custom node this would be accessed like this:

loc = self.inputs[0].get_location()

This feature would allow for custom link styles and colors and custom warning labels and other things.