fix(popup): _getEntry to search in headerEntries

[smbea]

Closes #695

[nikku]

Some observations here: The old popup menu actually used PopupMenu#trigger to trigger the event behavior. The new one does not do that anymore, effectively working around the public API.

Shall we adjust the new menu to use PopupMenu#trigger?

Other than that this PR looks good.

[smbea]

@nikku I'm not sure what you mean. The new popup menu still provides the PopupMenu#trigger API, see https://github.com/bpmn-io/diagram-js/blob/develop/lib/features/popup-menu/PopupMenu.js#L480. Or did I misunderstand what you meant?

[nikku]

GitHub messed up the deep link, let me inline it:

The old popup menu actually used PopupMenu#trigger internally, on entry click. The new one does not do that. Yes, the API still exists, but it is not used, under the hood.

My suggestion is now to consider restoring that behavior (if it makes sense).