Intertextual relations

[cwittern]

Is your feature request related to a problem? Please describe.
We can't describe intertextual relations yet. Let's try to do so. We call this a nexus list.
Describe the solution you'd like

• Intertextual relations are always defined originating from a 'current line', let's call it the baseline here (this is for example the line that is shown at the top of the attribution floater.
• In addition, a complete definition will need
  • a list of related text lines (target lines)
    • this might need to indicate additional lines from the target text, i.e. in a commentary where the linked line would be the source text which is commented on, which is then followed by additional lines, which form the content of the comment.
    • we want to have a note field here,
  • optionally, a 'key', which is for example a search term that is common to all text lines, or some other feature through which the lines are related
    • the key might have a field that acts as a definition, e.g. some text that might be displayed as a note and explains specifics of usage etc. seen here
  • A topic field, which will be used to group several such lists together

Additional context
A complete commentary for a text will have one such nexus list for every text line.