Add Read-Only mode

Implements a Read-Only mode in which entries and groups cannot be edited.

Note that entries and groups can be added in this mode, after doing so, the read-only mode is disabled and the user will have to set it again. This can be made a temporal read-write mode, but to do so, we require to turn Entry & Group pages into a GObject or Template.

Also note that it is also possible to delete entries in this mode.

Closes: https://gitlab.gnome.org/World/PasswordSafe/-/issues/86