Proof caching: external database

[FliegendeWurst]

This PR expands the proof caching infrastructure by introducing an external database of proofs.

Proofs are currently added manually by the user to the database. The user may inspect the database in a dialog.

TODO:

• [x] automatically add proofs to the database once they're closed (activated by an option)
• [x] automatically search in the external database during proof search
• [x] allow the user to remove proofs from the database in the UI
• [x] fix Java proof caching