feat: add multiple labels on elements

[vanfrancoisle]

Looking at the documentation, I noticed that there is no possibility to add multiple labels on elements like for arcs. I wonder if it would be possible to add multiple labels for elements in the library. The functions will be the same as for the arcs.
The labels are composed of a g element containing a rectangle and a text.

• Link to the source code of the arc label functions :

  joint/src/dia/Link.mjs

  Line 254 in 7771205

  // Labels API

• Link to the source code of an element : https://github.com/clientIO/joint/blob/77712052594632377b2e7124241f017abee3c943/src/dia/Element.mjs

Here is an image representing the functionality :

If this request is not possible, a solution is described in this url : https://groups.google.com/g/jointjs/c/r45SIrra1IQ
and another type of solution can be found in this discussion : #1749.

[kumilingus]

It would be possible to add an API that defines element labels separately from markup. And later implement features like label dragging. But that's a very low priority for now because you can:

1) define multiple labels already

Add them to the element markup:
https://jsfiddle.net/kumilingus/97dwxqv3/

2) drag labels

Add and embed them as new elements:
https://jsfiddle.net/kumilingus/zmb6hpy7/4/
https://jsfiddle.net/kumilingus/zmb6hpy7/

[vanfrancoisle]

Okay, thank you !