Make editor truly multi-root aware

[nikku]

Is your feature request related to a problem? Please describe.

As discussed with @marstamm there is the potential to clean up the root element vs. plane behavior and make the editor truly multi-root aware:

• We get rid of planes but rather allow multiple root elements to co-exist
• An business object (i.e. SubProcess in BPMN) may be both represented as a root element (with children) as well as an element (collapsed without children or expanded)
• Canvas APIs are simplified to accommodate this fact; we get rid of the leaky plane abstraction entirely.

This shall be a topic to investigate.

Describe the solution you'd like

const canvas = diagram.get('canvas');

// adding roots to a diagram
canvas.addRootElement({ id: 'Collaboration_plane', ... });
canvas.addRootElement({ id: 'SubProcess_plane', ... });
canvas.addRootElement({ id: 'SubProcessNested_plane', ... });

// deciding which root to display (hiding all other roots)
canvas.setRootElement(elementRegistry.get('Collaboration_plane'));

// removing a root element
canvas.removeRootElement(elementRegistry.get('SubProcessNested_plane'))

Describe alternatives you've considered

We can decide to keep the existing plane <-> root element duality at the cost of additional complexity, more APIs and so forth.

However, simply becoming multi-root aware seems the more sensible way to go in terms of reasonable API evolution.

Additional context

Related to bpmn-io/bpmn-js#1443