Reworked the NodeGraph to not require links and pins to have ids at a…

…ll in the data model.

Pins and Links have dynamic ids generated in the GUI now.
Protecting the operation list in NodeGraph so that the
node ids are generated by the NodeGraph and links are properly deleted when
an Operation is deleted that participates in those links.
Resetting the next node id when a NodeGraph is loaded so that
new nodes can be added without id conflicts.