Added new drawables and packed shapes for plugs on InputNodes and OutputNodes.

The input and output plugs on InputNodes and OutputNodes have special logic for drawing.
The InputNodeInputPlug is not shown on the InputNode.
The OutputNodeOutputPlug is not shown on the OutputNode.
This is because we don't want the user to modify these plugs because they
represent connections on the outside of the group which should not be
modified from inside the group.

These changes make these differences very explicit through these new classes.