Stable element IDs instead of indexes in the graph

In order to apply the Path node's vector modification on `Table` data, it is necessary to be able to identify which row it targets.

We could do this by index (e.g. modification to row 3 within the graphic table). However @Keavon wants to allow nodes to rearrange the table rows without impacting the result.

In order to do this, each row will have an id assigned when creating it. Nodes will be able to get their own id by having the input `TaggedValue::MyNodeId` (which would be populated during graph pre-processing). In order to avoid cutting and pasting changing the node id (as it does currently) some more logic to check if no such id exists will be added.

Each graphic table must contain unique ids. It is the job of the constructing node to enforce this (e.g. incrementing one of the ids to avoid collision).

Inizia individuando il nodo Path, la gestione di Table, TaggedValue::MyNodeId e il pre-processing del grafo descritti nell’issue. Il lavoro è completato quando le righe delle tabelle grafiche ricevono ID univoci e stabili, i nodi possono identificare la propria riga e il riordinamento o la copia dei nodi non modifica inaspettatamente tali ID.

Scritto dal modello di indicizzazione a partire dal testo della issue.