NetworkAgent: add support for VPN connections

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

src/shell-network-agent.h

@@ -9,6 +9,12 @@
 
 G_BEGIN_DECLS
 
+typedef enum {
+  SHELL_NETWORK_AGENT_CONFIRMED,
+  SHELL_NETWORK_AGENT_USER_CANCELED,
+  SHELL_NETWORK_AGENT_INTERNAL_ERROR
+} ShellNetworkAgentResponse;
+
 typedef struct _ShellNetworkAgent         ShellNetworkAgent;
 typedef struct _ShellNetworkAgentClass    ShellNetworkAgentClass;
 typedef struct _ShellNetworkAgentPrivate  ShellNetworkAgentPrivate;
@@ -45,7 +51,7 @@ void               shell_network_agent_set_password (ShellNetworkAgent *self,
                                                      gchar             *setting_value);
 void               shell_network_agent_respond      (ShellNetworkAgent *self,
                                                      gchar             *request_id,
-                                                     gboolean           canceled);
+                                                     ShellNetworkAgentResponse response);
 
 /* If these are kept in sync with nm-applet, secrets will be shared */
 #define SHELL_KEYRING_UUID_TAG "connection-uuid"