[Display Layouts] Should use composition API for adding and removing children, not mutation

[akhenry]

Composition changes (adding and removing children) should always use the composition API. Right now display layouts are directly mutating object composition. Our API allows composition providers to define and store composition any way they want (eg. on a server via web services), but unfortunately right now some of our views assume that composition is always held on an object.

Instead of directly mutating the composition property, developers should use the composition API like so:

openmct.composition.get(domainObject).remove(childObject);

[AliMuhammadOfficial]

I want to work on this patch. it would be honor for me to send PR.

[ozyx]

Testing Notes

1. Create a Display Layout
2. Add / remove objects to / from the layout
3. Save
4. Call this.openmct.composition.get(domainObject) in the console and verify that the composition is correct after each save
5. Reorder the objects in the Elements Pool
6. Save
7. Call this.openmct.composition.get(domainObject) in the console and verify that the order of the composition is correct
8. Repeat the same steps for Flexible Layout

[unlikelyzero]

Verified