Add create to palette

[smbea]

What should we do?

Add a "create anything" entry to the palette which provides a menu with all possible elements.

What it should look like (but with bpmn elements):

To do:

• Add entry to palette
• Add create menu entries provider

Why should we do it?

To support create anything, see #1627

[smbea]

When initially creating an empty pool, it is "casted" into an expanded pool. This makes sense generally. But (edge case) does it make sense if the canvas is empty?

Screen.Recording.2023-01-19.at.12.19.59.mov

We may want to leave it for now because it's not a usual sequence users take (i am assuming), but this could be tackled via CreateParticipantBehavior, in a way that it only happens if there are some elements on the canvas. I've tried this:

Screen.Recording.2023-01-19.at.12.18.30.mov

cc @nikku

[nikku]

But (edge case) does it make sense if the canvas is empty?

I think it does, because a diagram must always contain at least one process, and that is the one wrapped in the pool. We'd otherwise end up with empty pools in process diagrams (not containing expanded participants). We don't support such things yet.

[barmac]

An empty pool in a process diagram is a complex topic which we have not figured out yet. BPMN allows to have a collaboration diagram where one main process is not enclosed in a pool (participant), but there are no properties in the spec which would tell which process is the main one.