SLR: Add reasoning on databases/catalogs

[koppor]

A SLR contains a list of databases. In JabRef 5.7, there was enabled: false to document that a database was explicitly not selected. We removed that feature in JabRef#9123.

To "really" document the presence and absence of databases, an explicit field "reason" should be added to each catalog.

This has implicitations to

• the datamodel
• the UI

• Test if "old" study.yml files can also be read.
• Test if the "new" study.yml file can be read with a released version of JabRef.