Added edit and view modes to the nodes.

The right button on a node makes that node be the edit node for the node graph.
The left button on a node makes that node be the view node for the node graph.
Deletion of nodes also makes sure that it is removed from being referenced
as the view or edit node.

Node currently the group node also has edit and view,
which doesn't make too much sense. So we'll remove that at a later time.