Exposing a value to the graph should add and connect up an equivalent value node

Related: https://github.com/GraphiteEditor/Graphite/issues/2567

If you click the dot to expose an input to the graph...

(here, the top one is exposed and the other two are not)

...it removes the control from the Properties panel and puts a connector on the node in the graph.

But that connector starts out connected to nothing. And whatever value the user had typed in the Properties panel previously, isn't represented anymore.

We want to have the process of clicking to expose an input to create a value node in the graph which is set to the same value as the Properties panel had. (This will require the creation of new value nodes, since not all representable Properties panel types have a value node yet.)

As a stretch goal, we also would like the reverse to occur: if a user un-exposes a node, it should evaluate what its current value is and set that to the input in the Properties panel. Perhaps only optionally, though, because the current behavior might be desired some (most?) of the time. This aspect can wait to be figured out or we can break it out into a separate issue.