Simplify appending expanded sub-processes

[nikku]

Is your feature request related to a problem? Please describe.

When building a BPMN diagram, sometimes during the design phase I'd like to append an expanded sub-process directly from the current element. I'd even argue that, in the automation context, this is the more prominent action than appending a collapsed sub-process.

Right now though, this interaction is poorly supported. I.e. it takes an incredible amount of effort to get this done, even on an empty canvas:

Describe the solution you'd like

It should be easy to append an expanded sub-process right from the currently selected flow element.

• Update the append menu to allow appending an expanded sub-process or
• Update the replace menu to allow replacing a task to an expanded sub-process

Describe alternatives you've considered

As mentioned above, there is a few options.

Additional context

This is related to #1244, i.e. the fact that replacing a task with a sub-process (after it has been placed) results in unnecessary noise on the diagram. ➡️ It may be better to support appending the sub-process right away.

[philippfromme]

I've investigated this issue and there is one major roadblock making this a bit more effort to implement. The Modeling#appendShape API doesn't support multiple elements at the moment the way the Create feature does. Thus, appending a subprocess with a start event isn't as easy as adding the respective context pad entry.

[korthout]

I would really love this feature. Currently, there isn't even an option to expand a collapsed sub-process for C8 models. So I have to delete the element and drag it in from the side.

[nikku]

Will be solved via create/append anything:

[nikku]

Fully supported with create/append anything in bpmn-js@13+: