Allow DBusPropertiesMixinClass to be omitted from classes.
Allow DBusPropertiesMixinClass to be omitted from classes.

When it is missing, it is treated as if it were present but with no
static interfaces listed.

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib