Code cleanup and refactor for generalized layers

[adamgerhant]

Closes #1510, a follow-up to PR #1712.

Fixes undo bugs by extracting functions into messages. Generalized Copy and Paste for any layer node, and mapped duplicate layers (ctrl+D) to copy and paste.

Refactored Messages:
DocumentMessage::DuplicateSelectedLayers
DocumentMessage::GroupSelectedLayers
DocumentMessage::MoveSelectedLayersTo
DocumentMessage::UngroupSelectedLayers
NodeGraphMessage::DisconnectNodes
NodeGraphMessage::InsertNodeBetween
GraphOperationMessage::MoveSelectedSiblingsToChild
PortfolioMessage::Copy
PortfolioMessage::PasteSerializedData

New Messages:
NodeGraphMessage::DisconnectLayerFromStack
NodeGraphMessage::SetNodePosition
NodeGraphMessage::ShiftUpstream
GraphOperationMessage::AddNodesAsChild
GraphOperationMessage::DisconnectInput
GraphOperationMessage::InsertLayerAtStackIndex
GraphOperationMessage::InsertNodeBetween
GraphOperationMessage::MoveSelectedSiblingsToChild