Prevent loss of key ID when exporting in formats other than KDBX v4

[vnau]

When using KeePass2Trezor key provider plugin, each KeePass database is encrypted with its own key generated by Trezor. If the database is in KDBX v4 format, the key ID is stored in the file's public user data header. However, other database formats do not have such public, unencrypted data available until the database is decrypted. Thus, saving the database in other formats may result in the loss of the key ID and make it impossible to decrypt the database.

To avoid losing the key ID, it should also be stored in an auxiliary file, at least for all formats except KDBX v4.