GDBus: Add GDBusSignalFlags and use it in g_dbus_connection_signal_subscribe()

This is currently unused but will probably be useful in the
future. For example, we could have a _ARG0_IS_PATH to specify that
arg0 should be used for arg0path.

This commit breaks API and ABI. Users of
g_dbus_connection_signal_subscribe() will need to port to this new
version.

Signed-off-by: David Zeuthen <davidz@redhat.com>

gio/gdbusnamewatching.c

@@ -447,6 +447,7 @@ has_connection (Client *client)
                                                                                    "NameOwnerChanged",      /* signal */
                                                                                    "/org/freedesktop/DBus", /* path */
                                                                                    client->name,
+                                                                                   G_DBUS_SIGNAL_FLAGS_NONE,
                                                                                    on_name_owner_changed,
                                                                                    client,
                                                                                    NULL);