Handle nodes move and auto-layout on Python side by using GraphLayout and MoveNodeCommand.
* Node QML component relies on underlying core.Node object's position
* MoveNodeCommand is called after a Node has been dragged
* partial/full auto-layout for specific graph operations (Duplication, Augmentation...) are now part of those operations
* serialized position in node data allows to properly restore nodes coordinates on undo/redo
* remove all layout/node moving code from QML
handle this low-level operation engine side
* fix ListAttribute children links duplication
* handle CompatibilityNode duplication
* move corresponding unit test in test_graph.py
* [ui] add DuplicateNodeCommand
* bugfix: in latest PySide2 versions, application palette is not properly applied to all QtQuick Controls 2 components. Force this by exposing QApplication palette and bind it to the root ApplicationWindow.
* rename all "palette" ids to "activePalette" to avoid clashes with "palette" property on QtQuick Controls 2
* use parent component palette when it makes sense to propagate active/disabled style (instead of always using Active SystemPalette)
* allow duplication of one node/all nodes following a node
* actions available from Node contextual menu and Alt+Click on node for multi-nodes duplication
* add TextField to filter node types by text
* node can be created by pressing enter/return when highlighted
* give active focus to selected node
* node can be deleted when selected by pressing Delete key
* expose node types model as context property
* right clicking on an empty space inside the Graph Editor opens Node Creation menu with all available node types
