ModelElement

Common supertype of the Draw.io layer elements - what the format calls a cell and the Java API calls ModelElement: the things that carry a style, a link, tags, and custom properties. * NAMING - the Java API calls this ModelElement and reserves Element for the wider base (Document, Page, Model, Root, ModelElement). This file follows the shorter name; if the API name is preferred, rename here and in doc/readme.md together. * To be merged into drawio.xcore as a second supertype: * class DrawioLayer extends Layer, Element { … } class DrawioNode extends Node, Element { … } class DrawioConnection extends Connection, Element { … } * Ecore supports the resulting diamond (Node and Element both reach DiagramElement) without special handling.