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/tests/gdbus-threading.c

@@ -197,6 +197,7 @@ test_delivery_in_thread_func (gpointer _data)
                                                         "NameOwnerChanged",      /* member */
                                                         "/org/freedesktop/DBus", /* path */
                                                         NULL,
+                                                        G_DBUS_SIGNAL_FLAGS_NONE,
                                                         signal_handler,
                                                         &data,
                                                         NULL);