Links from document-level annotations do not show up on link targets

[reckart]

Describe the bug When a document-level annotation contains a slot feature and the slot filler is a span, then selecting the span says No links or relations connect to this annotation..

To Reproduce Steps to reproduce the behavior:

1. Create a document-level layer with a slot feature
2. Make a document-level annotation and fill a span into the slot
3. Select the slot-filling span
4. Problem 1: See "No links or relations connect to this annotation." in the annotation detail editor panel.
5. Delete the slot filling span
6. Problem 2: there is not warning dialog that annotations are attaching to the span (i.e the document-level annotation)

Expected behavior A back-link to the document-metadata annotation should be shown. It would be nice if we could even open the document metadata annotation from the span, but since opening the right sidebar at the right place may be a bit difficult, we should probably do without it.

Also, when deleting the slot-filling span, the usual dialog should pop up warning the user about the reference to the span being deleted.

Please complete the following information:

• Version and build ID: 0.19.6