refactor(dia.Graph)!: throw exception when cell constructor not found

[kumilingus]

Description

Throws an exception when a new cell is to be created from JSON if a type does not refer to a constructor.

This is to solve the recurring issue with dia.ElementView: markup required.

• The dia.ElementView: markup required error is thrown when a cell constructor is found, but the model defines no markup.

const MyElement = dia.Element.define('MyElement');
const graph = new dia.Graph({}, { cellNamespace: { MyElement }});
graph.addCell({ type: 'MyElement' });

const paper = new dia.Paper({ model: graph, frozen: true });
paper.unfreeze(); // throws `dia.ElementView: markup required`

• The new dia.Graph: Could not find cell constructor for type: 'MyElement'. Make sure to add the constructor to 'cellNamespace'. is thrown when the graph can not find a constructor for given cell type (e.g. when calling graph.fromJSON()).

const MyElement = dia.Element.define('MyElement');
const graph = new dia.Graph({}, { cellNamespace: { /* MyElement is not defined here */ }});
graph.addCell({ type: 'MyElement' }); // throws `dia.Graph: Could not find cell constructor...`

Migration guide

The PR also removes the default link type from the abstract class dia.Link to be aligned with dia.Element class model.

Before:

graph.addCell(new dia.Element); // throws `dia.Graph: cell type must be a string.`
graph.addCell(new dia.Link); // the link was successfully added to the graph

Now:

graph.addCell(new dia.Element); // throws `dia.Graph: cell type must be a string.`
graph.addCell(new dia.Link); // throws `dia.Graph: cell type must be a string.`

If you only work with models and don't intend to draw links or use a custom link view, you can add a type to the link.

graph.addCell(new dia.Link({ type: 'link' }));  // the link was successfully added to the graph

In other scenarios, please create a custom link (dia.Link.define('MyLink', /* ... */)) or use built-in links such as standard.Link.

[zbynekstara]

to fix layout-directed-graph tests do the following changes in joint-layout-directed-graph/test/index.js:

• graph = new joint.dia.Graph({}, { cellNamespace: joint.shapes }); - line 34
• change joint.dia.Link to joint.shapes.standard.Link - line 69, 70, 112, 113, 163, 164, 203, 204