Revert "Remove GnomeKeyringPasswords plugin"
This reverts commit b9fb89a3.
This plugin was removed due to the underlying library being deprecated and removed from some distributions. However:
- the gnome-keyring library is still available in Fedora Rawhide,
- the plugin is optional, and the availability of the library is automatically detected, so having the plugin available will not hurt users who do not have access to the underlying library, as Falkon will just be built without the optional plugin then,
- distribution packagers can put the plugin into an optional subpackage (which I have always done in Fedora to begin with),
- a replacement plugin using libsecret (the upstream replacement for libgnome-keyring) is still not available, and
- perhaps most importantly: not having the plugin available leaves users with no easy way to access their existing saved passwords (due to no libsecret plugin being available), which is IMHO not acceptable.