Feature/internal relations

[tijmenbaarda]

Add a form field to refer to another argument in the current library. The list is obtained in relations.js by scanning the Zotero library belonging to the selected annotation for Lidia annotations.

• PDFs without a parent item are not included in the list. I think this is fine because supporting PDFs without a parent item brings up additional problems (they have no bibliographic information attached)
• The Zotero keys are used as identifier. Are these guaranteed to be unique and persistent? (I think they are at least unique when working in a synchronized group library. We need to test if they remain the same when moving a PDF from a user library into a group library.)
• Only one relation can be defined. Do we need to support multiple relations?
• No support for reverse relations, although it would not be difficult to implement showing these.
• Do we need special support for symmetric relations?

Related to scalability:

• If there are many annotations, the combobox becomes too full
• The complete list of annotations is scanned every time the user selects an annotation, to guarantee that the user always sees all existing annotations. This may become too slow when there is a large library

We may solve these last two issues perhaps by providing a separate dialogue window for selection (with search functionality), but I would suggest leaving this for now so that the users can test.

Closes #28 (at least partially)

[ar-jan]

Do we need special support for symmetric relations?

I think we only have directional relationships (currently)? That also simplifies managing the relationships, since we don't have the benefit of referential integrity via a database. Maybe we should add an option to check whether all relationships are still valid.