fix(canvas): remove all planes on diagram.clear

[marstamm]

What is the problem?

• on diagram.clear the planes are not cleared
• Therefore, re-import of the same diagram is not possible if it tries to create the same plane again (related to BPMNImporter PR)
• Root elements are set to null on existing diagrams, but not populated with an implicit root by default (related to I have isn't working properties panel after release [email protected] dmn-js-properties-panel#23)

Fix
on diagram.clear:

• Remove all planes
• Reset active Plane

[nikku]

One small finding, looks good other than that.

[nikku]

One thing I'm asking myself: Why did our re-import tests in bpmn-js not catch the issue? Or don't we test that particular scenario?

If we don't I'd ask you to create an integration test in bpmn-js that does cover this scenario.

[marstamm]

The import and clearing/reimport works when we only use the default plane.
However, accessing the root element in the period between clear and reimport, we already had an active plane and therefore did not create a new root element, which lead to problems. Now we create a new plane with an implicit root, which is the same behavior as a clean canvas