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/gdbusconnection.h

@@ -453,6 +453,7 @@ guint            g_dbus_connection_signal_subscribe           (GDBusConnection
                                                                const gchar         *member,
                                                                const gchar         *object_path,
                                                                const gchar         *arg0,
+                                                               GDBusSignalFlags     flags,
                                                                GDBusSignalCallback  callback,
                                                                gpointer             user_data,
                                                                GDestroyNotify       user_data_free_func);