Improve collapsed sub-process support

[nikku]

What shall we do

I did update #87 to build on top of recent token simulation changes. The current state is that it does not break with bpmn-js@9, it does not work 100% properly either:

What does not work yet / shall be improved:

• Pause icon shows on sub-process plane => Do not show it
• No indication that tokens are active inside sub-process => Show them bouncing (cf. token on paused node) on sub-process activity
• Triggering events / event registration inside sub-process planes broken (Start events of nested event-subprocesses cannot be triggered #118)

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Originally posted by @nikku in #87 (comment)