Add ability to provide context pads for multiple elements

[nikku]

Adds the ability to create a context pad around more than one element:

• dedicated selection box surrounds all selected elements
  • implement
  • test
• a new context pad provider APIs allows provider to provide pads for more than one element (c.f. Add multi-element context pad 'delete' action bpmn-js#1525)
  • implement
  • test
• context pad is now single and multi-element aware. Target now generalized to djs.model.Base || djs.model.Base[] accordingly.
• even out popup menu entries so that image only entries are possible (366b5c9).
• fix selection not updating if selected elements changed (undo/redo)
• test isOpen api

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Capture of the thing in action for "delete" example (with reaction to undo/redo):

Screen.Recording.2022-05-18.at.12.34.00.mov

Delete can be tested by using bpmn-io/bpmn-js#1525 and linking this instance of diagram-js

Closes #278

[smbea]

I tested this with the updated diagram-js (on develop) and it works. Only requires a small change on the old getActivePlane function call. Will push the updated version and work from there.

It also works well with subprocess drilldown.

[marstamm]

Very cool. Tested it and am looking forward to have it in bpmn-js 🚀