Read only database file crashes XCA on save

[gl-dev]

If a database file is only accessible with "read-only" permissions in Windows, some changes are made and the database is saved, then XCA crashes.
Tested with v2.7

[tmontney]

Seeing this as well on 2.8.2-dev, Bookworm. Changes are not saved, as expected, but XCA crashes without error. If you run from the terminal, it complains about read-only.

1. XCA should warn about read-only upon opening the DB.
2. XCA should handle this gracefully or prevent the use of options which would perform write operations.