Distinguish unknown operators from obsolete ones (#2622)

With the ObsoleteAttribute removed from the placeholder type for an
unresolved operator, the editor no longer prefixes its description with
"This operator is obsolete".

The hatched node styling for unknown nodes is preserved through a
dedicated NodeFlags.Unknown, set from the UnknownTypeBuilder base type
instead of the removed attribute.

Closes #2372

Author: glopesdev

Bonsai.Core/WorkflowBuilder.cs

@@ -576,10 +576,7 @@ public Type ResolveType(Assembly assembly, string typeName, bool ignoreCase)
                         var errorMessage = string.Format(message, typeBuilder.FullName);
                         var descriptionAttributeConstructor = typeof(DescriptionAttribute).GetConstructor(new[] { typeof(string) });
                         var descriptionAttributeBuilder = new CustomAttributeBuilder(descriptionAttributeConstructor, new[] { errorMessage });
-                        var obsoleteAttributeConstructor = typeof(ObsoleteAttribute).GetConstructor(Type.EmptyTypes);
-                        var obsoleteAttributeBuilder = new CustomAttributeBuilder(obsoleteAttributeConstructor, new object[0]);
                         typeBuilder.SetCustomAttribute(descriptionAttributeBuilder);
-                        typeBuilder.SetCustomAttribute(obsoleteAttributeBuilder);
                         type = typeBuilder.CreateTypeInfo();
                         dynamicTypes.Add(typeName, type);
                     }

Bonsai.Editor/GraphModel/GraphNode.cs

@@ -44,6 +44,7 @@ public GraphNode(ExpressionBuilder value, int layer, IEnumerable<GraphEdge> succ
                 }
 
                 if (obsolete) Flags |= NodeFlags.Obsolete;
+                if (workflowElement is UnknownTypeBuilder) Flags |= NodeFlags.Unknown;
                 if (expressionBuilder.IsBuildDependency()) Flags |= NodeFlags.BuildDependency;
                 Category = elementCategoryAttribute.Category;
                 Icon = new ElementIcon(workflowElement);
@@ -124,7 +125,7 @@ public Brush ModifierBrush
         {
             get
             {
-                if ((Flags & NodeFlags.Obsolete) != 0) return ObsoleteBrush;
+                if ((Flags & (NodeFlags.Obsolete | NodeFlags.Unknown)) != 0) return ObsoleteBrush;
                 else if ((Flags & NodeFlags.Disabled) != 0) return DisabledBrush;
                 else return null;
             }
@@ -191,7 +192,8 @@ enum NodeFlags
             NestedScope = 0x10,
             NestedGroup = 0x20,
             Annotation = 0x40,
-            RangeUndefined = 0x80
+            RangeUndefined = 0x80,
+            Unknown = 0x100
         }
 
         static class CategoryColors