As a user I can copy and paste in two steps

[nikku]

Investigate, whether a two step paste could circumvent #1416.

Tasks

• To be pasted elements are displayed in the same way as create previews are displayed and can be position / snapped freely on the diagram
• Upon click elements are placed onto the designated target position

Open Issues

• after paste, all elements should be selected After pasting, all pasted elements should be selected bpmn-io/bpmn-js#1152 @pinussilvestrus
• multi-element paste breaks grid alignment Multi-element create breaks grid alignment bpmn-io/bpmn-js#1150 @philippfromme
• labels are lost during paste Labels get lost during pool extraction bpmn-io/bpmn-js#921 @philippfromme

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Related to #238
Required by #1416