Stunner - Issues when nesting BPMN containers

To issues found for BPMN lanes, but are generic for containers:

• Nesting BPMN swimlanes should be not allowed (which actually is)
• When copy/pasting nested lanes, some Sequence flows are missing. (see attached nested-lane-copy-issues) - Fix it in case of geeneric containers (as it have to be disabled for lanes...)
• Nested swimlanes appear on wrong locations after process reopen - Notice that if only affects to the BPMN marshallers, and nested lanes will be not allowed, this comment can be discarded

PR Acceptance test

As clarified with karreiro_, this issue focus just on lanes.

• Nested lanes not allowed
  • manually
  • copy paste
• Do some process that has event, gateway, subprocess and task in lane, reopen, check positions
• Copy and paste lane, save, reopen JBPM-7178