TODO

Ideally, the plugin should be able to open .gpg files transparently, decrypting them on opening and re-encrypting them with the same key when saving.
It is not clear to me how to implement this. One option could be to set the buffer insensitive and replace the text in it when doing both operations. But decryption should be done _after_ opening, and encryption _before_ saving.
Also: can a plugin support a MIME type and/or commandline options?

Translate strings with gettext.

If there are problems with seahorse (e.g. "dbus.exceptions.DBusException: org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.seahorse was not provided by any .service files"), the plugin should show a nice error dialog and not try to do anything, rather than failing silently.