Discard attribute changed callbacks during graph loading

[yann-lty]

Description

Ensure attribute changed callbacks are not triggered on graph loading.

When loading a graph, we expect nodes to be deserialized in the exact state they were saved.
Attribute callbacks may modify attribute values and impact the UID of the nodes.
This was leading to UID compatibility conflicts on file opening, for Graph containing Nodes using such mechanisms.

Features list

• Add "strictCompatibility" mode for graph loading
• Discard attribute callbacks when graph is being loaded

Implementation remarks

While this behavior was covered by a test, it turned out to be a false positive.
The error was hidden behind the fact that the tested Node was actually UID conflicting due to the callback, and swapped to a Compatibility Node. Therefore, attribute values were restored to the serialized ones, with expected values.
The introduction of the "strictCompatibility" mode allows to test this behavior more efficiently.

[waaake]

Did a test around this, the current changes did solve part of the problem which was loading a graph with no onAttrChanged callbacks triggered and no unwanted CompatibilityIssues raised.

My tests involved having a ChoiceParam Combo Box getting values populated in an onAttrChanged function,
and when the graph is saved and loaded back, the last value on the ChoiceParam is retained, but is shown in Red or invalid in the GUI, as the value is no longer part of the default values on the ChoiceParam which are loaded when the node gets deserialised.

Heart of the issue comes from the fact, we're not serialising the current set of values on the param in the graph file to be able to load them back upon loading.

Guessing this issue came into being with the introduction of allowing dynamic values on the ChoiceParam, as now the choiceParam can get updated values during any method calls.

Thinking about the best way would be to serialise values from such params.

[waaake]

Suggested change

	GraphCompatibilityError: If the Graph has node compatibility issues and `strictCompatibility` is False.
	GraphCompatibilityError: If the Graph has node compatibility issues and `strictCompatibility` is True.

I believe this should be True for the graph to raise an Error ?

[waaake]

To be able to reproduce what I did, please use the nodes from https://gist.github.com/waaake/d96a5ae6bd8f00c4a7ee0f99cd6a9242

Steps:

1. Create FirstNode.
2. Create Second Node.
3. Connect FirstNode.x -> SecondNode.y.
4. SecondNode.choice should now have the values as ["A", "B", "C", "D"] with the current value set as "D".
5. Change the value of this parameter to "B".
6. Save the graph.
7. Load the graph back.
8. Now the value on the choice parameter is set as "B" but marked invalid with a Red colour.
9. If we click on the choice parameter, the values are no longer present causing the current value on the param to be deemed as invalid.