[FEAT] Allow to display a called process by clicking on a call activity

[NathalieC]

Describe the solution you'd like We need the end-user to open a called process from a call activity in order to follow the case in all details. The called process should be displayed below the current diagram, in another browser tab, in a modal window. This should be configurable at the end. First iteration would be below, with the ability to collapse the "sub-diagram", expand it, and close it, since we can reach several levels of depth in the structure of the diagram while still give a good readability.

(quoting Thomas so as not to forget) "This can be done by a generic integration capability with BPMN elements (activities/tasks), either with mxGraph or otherwise. We need to be able to give the possibility to identify the elements directly, for anyone and not only in the dev team (using it already for rendering tests)."

[tbouffard]

mxgraph example with graph navigation (use the same graph, but update the model when clicking on a cell) https://github.com/jgraph/mxgraph2/blob/mxgraph-4_2_0/javascript/examples/offpage.html and https://jgraph.github.io/mxgraph/javascript/examples/offpage.html

[tbouffard]

We introduce a way to add custom behaviour/interaction to a BPMN element to manage what is described here: see https://github.com/process-analytics/bpmn-visualization-js/issues/857#issuecomment-737992557 that provides information about examples demonstrating this.

[tbouffard]

Improvements will be implemented with

• 927 to identify and access to call activities in various ways

• 929 to enrich data that we provide to navigate from the main process to the call activity (via the call activity process reference)

[tbouffard]

Examples and links to improvements have been provided in comments, so closing.