feat(popup-menu): allow entries to be draggable

[smbea]

Related to bpmn-io/bpmn-js#1802 -> bpmn-io/bpmn-js#1802 (comment)

[nikku]

@smbea I've extracted the "complex" test case into an integration test. It should be easier now to verify if the stuff works end-to-end (and how it works). Just it.only on a single test and a UI test bed is up and running.

Going back to #731 (comment), did you validate the actual feel? Any odd behaviors you found?

Do we assume that click is always provided, or can there be cases where only dragstart is provided?

I do like that draggable is set implicitly.

[nikku]

Is there a concrete reason to not mirror the ContextPad behavior?

The end result should be a robust, predictable behavior for the user.

[smbea]

@nikku I had another look at it and we can in fact have an almost identical behaviour there - sorry for the back and forth! The one thing I kept different was having the default value "click" for the action. This way we don't introduce a breaking change to the trigger API

[barmac]

Looks good overall. I left a couple of comments which you can consider to implement but IMO the PR is mergable as is.

[nikku]

UI wise always making the entries draggable and later canceling the default is the more stable behavior; it prevents ugly artifacts (text selection) as seen in the capture below:

I've updated the PR to that behavior (which we also have in context-pad, by the way).