Layer positioning improvements

[adamgerhant]

Improves the automatic positioning of nodes when moving layers in the layer panel

Todo:

• [x] Alt+click and drag to duplicate selection
• [x] Improve the undo/redo system to fix https://github.com/GraphiteEditor/Graphite/issues/1943
• [x] deleting should shift all nodes up by replacing single node selection with all upstream nodes
• [x] adding a layer should create space first
• [x] grouping artboard crashes

Deferred: Separating blue overlays from the rendering process to allow caching for boolean operations, prevent rendering the entire network when a layer is selected, solve https://github.com/GraphiteEditor/Graphite/issues/1889