[BUG] Elements of an expanded Call Activity are not linked to the Call Activity

[csouchet]

For such elements, the parent property is not set to the Call Activity.

If we filter a BPMN source to display only the content of a specific Pool (see #2063), the content of an expanded Call Activity is not displayed because we don't find the parent relationship like in this example involving the miwg-test-suite B.2.0 diagram.
This also prevents from collapsing call activities (we would like to restore this experimental feature #1871)

This is due to the fact that, at no time, the information from the called Process is stored: https://github.com/process-analytics/bpmn-visualization-js/pull/2020/files#diff-a3f4d22ee90476febc6e7329a9d846e235d02b8e5e49b9c3fdc9fe92c1937b38R151

To do that we need to

• store the converted CallActivity and its calledElement in a map
• store the converted elements of a Process and the Process id in a map
• when all the processes are converted, assign the CallActivity as parent of all the elements of the Process corresponding to its calledElement.
• review unit and integration model tests to ensure we check the inner elements of the expanded CallActivity
• update/add e2e visual tests to check that inner elements are displayed (do something similar to what we have for expanded sub-process)