Simplify canvas widgets placement at one or multiple specific layers

[AlexeyMz]

• Add CanvasPlaceAt component to render a widget at specified non-viewport canvas layer;
  • Now it is possible to place widgets at underlay and overLinkGeometry layers;
• Add changeTransform event to CanvasApi.events which triggers on CanvasApi.metrics.getTransform() changes;
• Add --reactodia-z-index-base CSS property to override base z-index level for workspace components with a z-index set;
• Fix HaloLink and visual authoring path highlight being rendered on top on elements by placing it onto overLinkGeometry layer;
• Deprecate canvasWidgets prop on DefaultWorkspace and ClassicWorkspace in favor of passing widgets directly as children;
• [Breaking] Remove defineCanvasWidget() and SharedCanvasState.setCanvasWidget():
  • Canvas children are always assumed to be viewport widgets;
  • Use child CanvasPlaceAt components to render different parts at other layers instead.
• [Breaking] Canvas widgets are not re-rendered when parent canvas is rendered and require explicit subscriptions:
  • Subscribe to canvas changeTransform event when using CanvasApi.metrics to convert between coordinates;
  • Subscribe to canvas resize event to track viewport size;
  • Subscribe to changeCells event from DiagramModel to track graph content changes.