VPN secrets are stored by the plugins, that provide separate
helpers for authentication. This commit adds the support for invoking
the binaries and pass them connection details.
For plugins that support it (as exposed by their keyfile), we invoke
them in "external-ui-mode" and expect a set of metadata about the
secrets which is used to build a shell styled dialog.
https://bugzilla.gnome.org/show_bug.cgi?id=658484
A network agent is a component that stores network secrets (like
wifi passwords) in the session keyring. This commit adds an
implementation of it to be used by the shell network dialogs. It
handles most of the keyring stuff, delegating the UI to upper layers.
https://bugzilla.gnome.org/show_bug.cgi?id=650244
