Add node to graph with Ctrl+V if valid node content is in the clipboard

Attempt to paste the clipboard's content in the graph when Ctrl+V is
pressed. If the clipboard contains a valid node description, add the
corresponding node to the graph.
Otherwise, do nothing.

meshroom/core/graph.py

@@ -396,6 +396,15 @@ class Graph(BaseObject):
 
         return duplicates
 
+    def pasteNode(self, nodeType, **kwargs):
+        name = self._createUniqueNodeName(nodeType)
+        node = None
+        with GraphModification(self):
+            node = Node(nodeType, **kwargs)
+            self._addNode(node, name)
+            self._applyExpr()
+        return node
+
     def outEdges(self, attribute):
         """ Return the list of edges starting from the given attribute """
         # type: (Attribute,) -> [Edge]