[core] Trigger node desc creation callback only for explicit node creation

The desc.Node.onCreated callback is a hook in the Node API for developers to
write custom behavior on a node first initialization.
By being called in the core.Node constructor, this was triggered in several situations
that don't match this idea and with unpredictable side effects (graph loading, node re-creation on undo...).

* Make `onNodeCreated` callback an explicit method on desc.Node.
* Remove the call to node descriptor's `onNodeCreated` callback outside core.Node constructor.
* Trigger this callback on explicit node creation (adding new node to graph, init a graph from a template).

tests/test_nodeCallbacks.py

@@ -0,0 +1,68 @@
+from meshroom.core import desc, registerNodeType, unregisterNodeType
+from meshroom.core.node import Node
+from meshroom.core.graph import Graph, loadGraph
+
+
+class NodeWithCreationCallback(desc.InputNode):
+    """Node defining an 'onNodeCreated' callback, triggered a new node is added to a Graph."""
+
+    inputs = [
+        desc.BoolParam(
+            name="triggered",
+            label="Triggered",
+            description="Attribute impacted by the `onNodeCreated` callback",
+            value=False,
+        ),
+    ]
+
+    @classmethod
+    def onNodeCreated(cls, node: Node):
+        """Triggered when a new node is created within a Graph."""
+        node.triggered.value = True
+
+
+class TestNodeCreationCallback:
+    @classmethod
+    def setup_class(cls):
+        registerNodeType(NodeWithCreationCallback)
+
+    @classmethod
+    def teardown_class(cls):
+        unregisterNodeType(NodeWithCreationCallback)
+
+    def test_notTriggeredOnNodeInstantiation(self):
+        node = Node(NodeWithCreationCallback.__name__)
+        assert node.triggered.value is False
+
+    def test_triggeredOnNewNodeCreationInGraph(self):
+        graph = Graph("")
+        node = graph.addNewNode(NodeWithCreationCallback.__name__)
+        assert node.triggered.value is True
+
+    def test_notTriggeredOnNodeDuplication(self):
+        graph = Graph("")
+        node = graph.addNewNode(NodeWithCreationCallback.__name__)
+        node.triggered.resetToDefaultValue()
+
+        duplicates = graph.duplicateNodes([node])
+        assert duplicates[node][0].triggered.value is False
+
+    def test_notTriggeredOnGraphLoad(self, graphSavedOnDisk):
+        graph: Graph = graphSavedOnDisk
+        node = graph.addNewNode(NodeWithCreationCallback.__name__)
+        node.triggered.resetToDefaultValue()
+        graph.save()
+
+        loadedGraph = loadGraph(graph.filepath)
+        assert loadedGraph.node(node.name).triggered.value is False
+
+    def test_triggeredOnGraphInitializationFromTemplate(self, graphSavedOnDisk):
+        graph: Graph = graphSavedOnDisk
+        node = graph.addNewNode(NodeWithCreationCallback.__name__)
+        node.triggered.resetToDefaultValue()
+        graph.save(template=True)
+
+        graphFromTemplate = Graph("")
+        graphFromTemplate.initFromTemplate(graph.filepath)
+
+        assert graphFromTemplate.node(node.name).triggered.value is True