telepathyClient: ignore invalidated channels

There is a race if a channel is invalidated during its preparation: the
'invalidated' signal is already emitted so the Shell will never notice.
We fix this by simply checking if the channel is already invalidated when
receiving it from telepathy-glib.

In the approving case, we reject the full ChannelDispatchOperation as we only
support approving one channel at the time.

https://bugzilla.gnome.org/show_bug.cgi?id=677457

src/shell-tp-client.h

@@ -105,5 +105,7 @@ void shell_tp_client_grab_contact_list_changed (ShellTpClient *self,
 void shell_decline_dispatch_op (TpAddDispatchOperationContext *context,
                                 const gchar *message);
 
+gboolean shell_is_channel_invalidated (TpChannel *channel);
+
 G_END_DECLS
 #endif /* __SHELL_TP_CLIENT_H__ */